Gonna be brutally honest here... 8 ball is an easier game and there's a good chance you've convinced yourself you're playing the two games completely differently when in reality your playing level is about the same.
You just get out less in 9 ball so that has you convinced you are playing worse. But your skill level hasn't changed.
What I think is happening in your 8 ball game: If you screw up position, you shoot a different ball. You might act like you meant to play that ball next and your opponent might buy it, and neither of you realize you're getting away with murder every time you mess up shape but get to keep shooting anyway. In 9 ball you can't do that.
As a result your average run in 8 ball is much higher, and you will occasionally run out of racks, but 9b stumps you because you are not allowed to mess up position and you will need the accuracy, planning and stroke required to shoot a ball at one end of the table and get to the other end. If you lack those, your run ends after a few balls. It's not because your shotmaking ability drops by 5 balls merely from switching to a different game.
If your shotmaking IS decreasing, it's because you can't sink balls in an effortless, automatic way anymore, now that you're forced to divert some attention to moving whitey along some long, unfamiliar routes (like zigzagging across the table or playing 3 rail shape after a rail cut).
Sorry in advance if I've got you all wrong! But I truly believe this is what's happening. Even high B players can look like low C's in 9 ball and will have lower average runs and fewer runouts.
