Why should umps have different strike zones?

I'm watching the Giants/Phillies game and already one of my big pet peeves in baseball has reared its ugly head.Dan Iassogna, the home plate umpire, called ball four on Jimmy Rollins to drive in the lone run of the game so far. It looked knee high down the center of the plate.An inning late, he called a pitch lower than the one thrown to Rollins a called third strike on Fontenot to end the