Sounds strange to me. Could it possibly be to add in 6 inch layers- i.e. 6 in. layer nitrogen, 6 in. layer carbon, alternating? That would be my guess.

I doubt that it would ever heat if you added only, say, a 6 inch layer of grass per week. I like to fill the whole bin at once when possible.

Karen

Don't know what kind of bin you're referring to, since you didn't provide any information about the make or brand.
Assuming that it is the 'rotating' kind on a stand, the recommendation refers to the procedure to attain a proper MIXING of 'ingredients' before adding more - because adding too much to the rotating-type bin at one time, will cause difficulty in attainting a complete mix of materials.
Sounds like the material previously added into the bin were deficient in green (nitrogen) materials which later were added to the bin via fresh grass clippings.
In that case, you were 'lucky', due to insufficient moisture in the original mixture - which was added with the green (not aged) grass clippings).
Grass clippings should be 'aired' (spread-out) at no more than 12" deep) for at least 24 hours (and 3 days is the recommended time) before adding to a 'rotating' bin enclosure/containment to avoid anaerobic conditions (slimey mess) in the bin.
Recommended ratio of 'brown' (carbon) materials to 'green' (nitrogen) materials is between 20:1 to 30:1 depending on who is making the recommendation. By weight, not volume.
Agree with kqcrna - my preference is to fill the rotating container in one session - 6" deep layer (in the middle of the round bin) at a time, then rotate/mix at least 30 SLOW revolutions - then add more and rotate/mix again.
IF the grass clippings are properly 'aged' (i.e., heaped to make a ground pile initially), then used that pile in the bin next, while making another pile with mowed clippings to 'age' for a week or two - then the grass clippings are ALL you need to produce good compost for a garden or flower bed as mulch (surface) or amendment (turned-under soil). Sufficiently 'aged' grass will have a very good carbon-to-nitrogen (C:N) ration all by itself - if it has been properly fertilized and watered prior to mowing. Reach in and grab a handful of what's in the bin.
Look at it. Smell it. Squeeze it. If it's not turning brown, or doesn't smell 'earthy' and if water drips out from your clenched fist - make approptiate adjustment. The mixture should be 'damp' but no water should drip out.
Contrary to opinion of some folks, manure and leaves are NOT absolutely necessary ingredients to make quality compost - but I always use both, when available. Better, yes - but not essential. But DO remember to add a shovel-full of previous fresh FINISHED compost to the next batch -or if a first-time mix, add a shovel-full of the 'best' dirt (excluding woody mulch) in your bin - to add microbe populations to begin with. Compost leafy materials, not 'woody' (cellulose/lignin) materials. No oil, no meat (fish is OK)., and no bones. Those encourage population shifts of the WRONG microbes to produce quality compost.
IF you started with kitchen scraps and 'other stuff' initially, chances are that the microbes that decompose organic material were simply in very short supply to begin with - and even with proper levels of air/food/water, it takes awhile (3-5 days) for microbe populations to expand sufficntly to make a difference you can see. Particularly thermophyllic fungi, which cause a sufficient mass of organic material to heat up (which are naturally present in fresh grass clippings). They need a high content of sugars and carbohydrates (+ air and water) for their population to 'explode' sufficiently to produce a significant heat (130F is about the maximum temperature you can expect from a 'rotating-type' bin due to its limited biomass). DO buy/use a compost thermometer.
Recommend you read some information authored by Malcom Beck, considered the "King of Compost". I don't agree with everything he writes - but then, only idiots do. He writes for the masses. I'm writing only to you. Become self-educated about composting techniques.
Experiment and learn what works for YOU.
Basically, there are only 6 items necessary to produce a high quantity of quality compost: 1) microbe populations; 2) air (oxygen) for microbe survival; 3) food for microbe growth; 4) water for microbe reproduction; 5) your time and 6) your energy.
Provide those things in sufficient quantity with efficient timing and you'll produce quality (% humus content and high plant nutrition level) every batch.
There are basically two composting techniques: 1) continuous-add (CA) method and 2) finish-to-harvest (FH) method. CA keeps the pile 'warm' after initial heat, and FH allows mesophyllic organisms to 'finish' the pile (don't add any more ingredients) - which maximizes humus (carbon) production. In science, HUMUS is defined as the product resulting from decompositon that will NO LONGER decompose. Some humus has been found to be over 3,000 years old.
I've been composing for over 50 years and am a certified Master Gardener and certified rainwater harvest professional. I recommend rainwater for compost - because to the degree you add chlorinated water to compost - is the degree that you kill composting organisms and defeat your purpose for composting. Chlorine is an VERY effective microbe killer - which is why city muncipal water systems use it to disinfect public water - which is usually downstream of an upstream city's pollution.
DO collect compost 'tea' - even from watering your rotating bin material. It's "liquid fertilizer" and chock full of beneficial microbes for your soil.

There is no dumb question, except the one never asked because someone thought it was a dumb question.
I think you may be misinterpreting what was said about that 6 inches of material. Very often, in the fall, I will build a compost pile in one day that is 4 x 4 x 4 but for each 6 inch layer of leaves I add the appropriate amount of a Nitrogen material. Sir Albert Howards original recipe for compost was a 6 inch layer of vegetative waste, a 2 inch layer of manure, and 1/8 inch of good, rich soil thinking that the soil bacteria would be what digested that material. Today we know that is not necessary since all waste has the organisms necessary to digest that if they are given the right environment to work in.

I think the instructions are indeed referring to single materials.

I often mix up a cubic yard size batch of grass clippings and leaves on a big tarp by grabbing the corner and pulling it one way then the other. I can fill and entire bin this way with a perfect mixture in one swoop, and it will heat up. Fall is a great time to do this, at the last mowing which sucks up both leaves and grass clippings. Or in the spring I use stored bags of leaves. This works great and there are no ill effects of that much at once.