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My sense of 'normal' vs. 'out-of-bounds' makes me think it probably isn't the centered FB that's causing the extra hits. If it were obvious (and fixable), I'd think somebody prior to Seattle would've already remedied the problem. So, my guess would be that the culprit is something subtle - that gets shrugged off, since it's a come-and-go issue.
In thinking about the Unsub - the savior for Fister 'might' be that he DOES isolate the occurence -- so maybe he'll actually be an 8.4 hit pitcher for Seattle 70% of the time - and the hit-parade games are spread out - and are therefore categorized as 'just an off day'. The key in that respect would be to try and identify a tell for those days - and when they occur, go with a really quick hook.