Why is a 59 OVR player capable of rushing for 1200 yds consistently?
Sounds like something other than progression is at work for that. Of course, judging players by OVR is...inexact at best.
Especially if the weaknesses of the player's skill set do not translate onto the field. So if his OVR is low because his AWR and BCV are low, but you're controlling the HB, then the things that make him a 59 are not going to show up nearly as much because his BCV becomes your eyes that can, literally given the gameplay view, see the whole field and his AWR is your reaction time and your football knowledge and your skill at hitting buttons and the sticks, etc.
And in real life - there's a lot more dynamics that goes into a player's performance. A player can indeed be a very average HB, but if he has a good line and a good blocking scheme, he can put up higher tier numbers.
Plus, one year does not a career make. I looked up some of Denver's HB during Shannahan's time there and found guys with 1100 yd seasons...and never did anything close to that ever again. I'm sure there's plenty of other examples. Should that guy get some high rating for 1 year, and then perform at that level always? Sounds like these guys would be perfect examples of scrub HB that had one good year, but are inconsistent-at-best in terms of real ability.