I also find that hard to understand given your settings (I'm assuming that varsity is easier than all-american). I play on the 360 on AA. I started with Jistic's sliders and made some modifications along the way. My sliders should make it much much MUCH easier for the CPU to run against me than it is against you.
your_settings(my_settings)
difficulty level: varsity(all-american)
human break block: 75(~20)
human tackling: 50(~30)
cpu run block: 45(~75)
cpu RB ability: ?(85)
As you can see, you should break a TON more blocks than my guys, my cpu opponents should run block MUCH better than your cpu opponents, your guys should tackle a LOT better than my guys and I would assume that my cpu opponent's RBs are much better runners than your opponents' RBs. But, I rarely have difficulty stopping the run. Like you, I'm pretty bad on defense which is why I had to stop playing the MLB (my favorite defensive position) and take over the SS. I lead my team in tackles sometimes, but my cpu controlled teammates make the lion's share of the tackles so it's not like it's my ubAr skillz that are the difference as most people would probably like other people to believe. It's my computer controlled guys tackling other computer controlled guys most of the time. I play as Michigan (who has a great defense) and North Carolina (who has a WEAK defense) and neither of them have much trouble with the run. Carolina obviously has more 100 yd rushers (and 200 yd rushers
)against them than Michigan, but even Carolina doesn't struggle like you're saying you do (with Oklahoma no less).
Do you play on the 360? If not, maybe it's just a system difference, but I would doubt it would be that profound.
What kind of D do you play against the run? Are you in dime coverage every down or something?