Umpires tend to position themselves over the inside part of the plate… putting them at a disadvantage when calling outside pitches. We witness the same effect when it comes to the height of the pitch: the farther away from the ump's eye, the more likely he will make a call that is inconsistent with that of the Pitch F/X system.
While the off-speed zone is clearly larger than the fastball zone, it is also higher. So while off-speed pitches are gaining more favor in the top third of the zone, they lose favor in the bottom third (which sees over 50% more pitches).
Right-handed batters are significantly advantaged in terms of the strike zone. Take a look at how much the zone shrinks when a righty comes to the plate.
