I agree there's some randomness involved, but as long as I'm able to hit the ranges I've figured out per player, I've never had a problem other than wavering between "strong" and "proficient", which isn't a back breaker.
I've noticed the same thing relating to stamina. There seems to be something similar in substitution patterns as well- whenever I have a backup covering two spots, he invariably subs in for the lower stamina player more, regardless of their relative skill level. Likewise, when I play with a unique backup at each position, the players with lower stamina get subbed out more frequently than my high stamina guys. I can't imagine they'd have that happen in-game without game shape being in part by stamina.