The sliders aren't the end all of what is going to happen so even though something is positive or negative doesn't necessarily mean it will occur less. You still have to do what is right to get that to happen. You could set the cpu power at -50 and if you groove fastballs right over the plate all day, they'll still hit a good many for HR's (which they should).
HR's is purely based on good timing in this game. Your power and contact can be set to 50, but if you don't time the pitch perfectly (and I mean perfectly), you won't do squat with it.
The CPU can hit them out in the negative because they time almost everything perfectly. You really have to waste a bunch of pitches to throw them off.
The AI is MUCH, MUCH smarter on All Star than Pro. Even smarter on MVP Level. So when you jump up a level, you won't get away with some, or many of the things that you got away with on the previous level. Might not even have realized what you did on Pro wasn't very sound strategy in terms of pitching/setting up hitters, but if it wasn't, they'll make you pay on All Star.