Matchup affects it. That's why your players will always have better ratings than normal against bot/tanking teams and slightly lower when moving up divisions.
Also, skills are weighted higher at different positions to determine rating. For example, guards will have a much lower rating when playing the SF, because they tend to have low ISP. But if your running R&G, and the wings are shooting more threes, then the rating matters less.
I think rating is more useful as a guideline for trying to determine what your opponent's players' skills might be.