If you think of the possible outcomes as a progression, it makes more sense.
Level 1
Walk-AB over
Strikeout-AB over
HBP-AB over
Ball in play-Move to level 2
Level 2
Hit-Move to level 3(what kind of hit?)
Out-AB over
Error-Move to Level 3a(what kind of error/how many bases?)
And so on. A guy with high contact is going to move to level 2 a lot more often than a guy with below average contact.