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Most likely is that he's simply taking a little off, since overthrowing doesn't necessarily take -5 off your fastball.
Problemo is, that can result in 'steering' the ball -- like a golfer DECELERATING the club through the ball -- and far worse mechanics. When you're not accelerating through the release point, you lose the centrifugal 'trueness.'
We only mention it, because that's what Brandon Morrow is in fact doing.... :- )
The only solution is for somebody to find the variability in the plant foot, or whatever the variation is. You'd think with 3-D spline video that work would be a piece of cake these days, wouldn't you?