What ROGER posted here is 100% on the money. And if all the things that he mentioned aren't enough... these things can change inning to innnig, and even pitch by pitch.
That's what makes this positon (pitcher) so difficult to train for. And as you progress into the higher more competitve levels .. the pace of things really picks up and the dynamics of multiple situations compounds the challenge.
Also, don't forget .. the guy holding the bat also injects a little adjusting of his own. For example... suppose a batter that your were facing in the second innning, show up in the fifth inning with a shorter bat. Will you notice it ? Nope. Will your backstop notice it? Nope. Well, if neither one of you picks up on it .. whats the big deal.
The big deal is this.. if you faced a batter in one inning with say a 34" length.. then the same batter steps in with a 32" lenght, he's just drawn th sweet spot of the bat about 2" closer to him. So, if your going to box-em inside.. and you miss by just 4 or 5 inches on your target location.. this batter has some pretty good odds of makeing contact. That contact by the way is going to entirely different than what you may have seen in prior innings.
Try it for yourself and you'll see what I mean. Start off with a batting T, and try various bat lengths to see where the ball goes using the same batting stance and body posture when swinging. Just remember to leave the batting T setup in the same spot.