Your perception was that the pitcher, especially with a pull rating of 1, should always pitch to his MPC if he's not in any sort of trouble. You saw the TPC-MPC gap as a range that the pticher should be between when he is eventually pulled from a game.
Instead, think of TPC as the number of pitches you WANT your pitcher to throw in an ideal game, and the MPC of how much you will ALLOW him to throw, should he exceed his TPC mid-inning (especially if he is getting into a jam).
If he has finished an inning and cannot realistically be expected to finish off a batter in the gap between his PC and TPC, the sim will pull him. The MPC only comes into play mid-inning. If you want to ensure he throws 80 pitches for example, set your TPC to 85.