Another factor is what else do your chickens eat? A chicken in a run with a sand/dirt floor that is not fed treats/veggies/etc. is almost exclusively eating the feed provided and getting the full percentage of protein.
A chicken fed other things or given access to forage for plants/bugs/seeds/etc. and has a more varied diet may be inadvertently diluting (or even increasing) the amount of protein they get in a day because they are not just eating the feed.
A chicken that is not kept in a climate controlled environment that experiences a wide range of ambient temps in the year may be using their nutrition for a lot more than just egg production.
The same goes for a chicken that is not actively laying, molt, illness, stress, age, etc.
There are a lot of factors that impact the nutrition a bird actually needs and agricultural nutritionists have done their best to come up with a one size fits most feed formula. So we find ourselves with 16% layer feed for backyard flocks. It's not perfect, but it's a great starting point.