The only thing I can think of is that with a drop spreader, it's too easy to get stripes. Unless you line up perfectly every time, you'll either get a little overlap, which gives you darker green stripes where the passes overlapped or you'll have slight gaps, which gives you lighter green stripes where you missed.

You can try to counter that by making two passes in different directions, but that can give you a checkerboard effect. When I bought my house, I had stripes, then a checkerboard, then a broadcast spreader.

bpgreen has a good point, but that can happen with any fertilizer applied with a drop spreader.

Im almost positive the main reason is because it has a coating on it. (slow release i think) When you use a drop spreader the coating may get cracked or broken. In turn this could result in burning, because the nitrogen is released too fast

Another possibility (although one of our more energetic members will likely track down the Lesco scientist who made that recommendation, and find out why) might be grain size.

Imagine a fert mix that you and I combine with kid's marbles, and throw them into a spreader. Then spread on the lawn. With a drop spreader, and those narrow slots, all the marbles will be left in the hopper, and all the fert will be on the lawn. With a rotary spreader, that slot opening at the bottom is quite large, and I imagine that both fert and marbles will be all over the lawn, because the opening was bigger.

So the bigger grains might not go through, or might not go through as well on a dirty spreader, or clogged slots, etc. I have no facts to back this up, it's just a guess.

Here's another theory. If the Lesco products in question are high N synthetic, a broadcast spreader will be less likely to stripe, like BPGreen wrote. Low N fertilizers (especially organic) are less likely to stripe than high N fertilizers even when applied with a drop spreader.

-Deerslayer

It is very simple, that it is what it is engineered for. The size and shape (spherical) is designed for rotary broadcast spreader. Keep in mind Lesco products are for professional use. Pros do not want to waist time applying fertilizers like you would mow as it is very inefficient use of time.