My first guess would be difficulty/sliders. At the higher difficulties, user sliders start out lower (CPU sliders higher), which directly impacts your (and CPU) players ratings for those specific areas (i.e. shooting, rebounding, whatever). There's a thread floating around here somewhere that details the penalties (bonuses for the CPU) of the higher difficulties.
It's the main reason why I use
JasonWilliams55's HOF sliders, as he made adjustments so that the user penalties and computer bonuses were nullified as much as possible, thus letting the ratings dictate how the players perform.