It's interesting to look at data:


What you can see from the charts, you can say:
(1) the strike zone varies from umpire to umpire, offset in some "systematic" way, e.g., Welke's zone is systematically wider on the outside than McClelland. (in fact they often vary by handedness of pitcher and also the pitcher himself)
(2) near the edge of the strike zone, the pitches can often go either way, and it's basically a coin flip.
(3) they are rare, but there are indeed blatant missed call, even well inside/outside the strike zone.
So the fact that the call seems to be fairly random near the edge of the strike zone, almost as random as coin flip, is just a reflection of reality.
What I don't notice strongly is the umpire to umpire tendency, the systematic "overall offset" of strike zone. I'm sure it's coded in, but I'm yet to find out the clear tendency myself, even after all these games. For example, as a pitcher, I don't feel there is any umpire with which I can consistently get favorable call on pitches just outside of the zone on my way.
The images were borrowed from this article: http://www.baseballprospectus.com/ar...rticleid=14951
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