perhaps because tags are made by the marketers ... and not the horticulturists ...

best info is usually had by using latin names .... getting you to horticultural sources ...

look for annual growth rate .. and do your own multiplication ...

do note.. most things dont stop growing at some magical height ...

and then you have to add in where you are .. to the equation .... e.g. in my MI you have a basic 4 month growing season ... with a couple cold months on either side [slow motion growth if any].. with 3 months of ground freeze ... comparing to further down south.. you might have an 8 month growing season .. with barely any dormancy ... but for the declination of the sun ... etc ... [ i know.. it doesnt add up to a 12 month year.. lol .. that other 30 days is called.. the vagaries of mother nature .. lol ...perhaps a long winter vs a short winter.. etc] ...

ken

Plants will grow more quickly in regions to which they are best adapted. So, in my zone I would get much slower growth in say a live oak than you'd see in the south. So there's that.

If azaleas and rhodies do well in your region, then I'd plan on the larger size. It can be very hard to visualize a small nursery plant reaching full size. Probably the most common gardener's mistake is planting things too close together.

Probably the most common gardener's mistake is planting things too close together.

==>> or too close to the foundation with no concept of future potential

Ken usually says "because the plants don't read the guidebooks" which is 100% true. It all depends on weather, sun, zone, water, etc etc. As the pirates say, it's more of a guideline.

Plant tags tend to be generically created and so attached to plants that are sold and grown in vastly different locations and climates. The sizing often reflects a average size as a result.....the same plant may not grow as large in some harsher locations or grow much larger in a more beneficial location.

Most also reflect an estimated size at a given point in time. It could be 5 years, 10 years or 20 years. Obviously many plants can live much longer than these arbitrary yardsticks and therefore could grow potentially much larger, but these time frames are typically chosen to coincide with how frequently a house changes hands and a new landscape is installed or an old one renovated.

Plant sizes on tags or labels are merely guidelines. Better references will generally present a range at 'maturity'.......closer but still a nebulous marker :-) Your best source of info is going to be local sources, like extension service publications. Or first hand experience.

I typically visit a botanic garden, either local or on line. My favorite on line is the Missouri Botanic Garden’s Plantfinder because the estimates of size often reflect what I see for plants that aren’t on the edge for hardiness here.

Another factor is that people like to be told what they want to hear. Most people want to hear that a plant will not get too big. So let's tell them that it will be 4' instead of 8' and they'll be more likely to buy it.

Slow growing yet long lived shrubs like boxwood, rhododendrons, dwarf conifers and lace leaf maples just keep getting bigger and bigger as the decades go by. So that if a body is on a property long enough they may see the one they planted way back in _ get as big as a car. Or bigger.

Many factors will determine the mature size of any tree or shrub. The most obvious influence on plant size is genetic makeup – you only have to look at cultivar names like ‘Midget’ or ‘Giant Candles’ to understand this component. Geographic location also plays a major role in determining height. For instance, trees tend to grow taller in areas where temperatures are more moderate; trees in coastal areas are generally larger than these same species in more continental regions. Within a geographic range, local climate will further influence final size: rainfall and temperature can vary widely within a region as a result of local topography. The microclimate of a site will influence tree size due to differences in environmental factors such as drainage, soil type, and aspect. Finally, competition for water, light, nutrients, and other essentials will affect not only growth rate but final height as well

The Myth of Uniform Plant Performance

https://s3.wp.wsu.edu/uploads/sites/403/2015/03/tree-heights.pdf