Some players come in as freshmen and are already halfway maxed out due to their rigorous HS strength programs (which are sometimes at private "performance centers", not at the high school).
One of the keys to successful recruiting is to identify those players and recognize that their upside is not too high and to try and find diamonds in the rough that will improve a lot.
And, yes, if you take 100 18 year olds that are, say 6-2, 210, with similar body types and put them through the same two-year regimen, you will get significant variances in added strength, weight increase and changed looks. Part of the reason for that is that some of them may just be going through the motions, but the main reason is that individuals respond differently to strength training.
Also, responding to one part of this thread, I feel reasonably certain that PEDs are fairly common at all levels of football (post-Pop Warner, at least).