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There is a lot of luck involved, as to which one is going to take a step forward. You've got 100, 200, 300 pitchers around of Blackburn's/Gaviglio's ability. Some small portion of them will get better. It's like Pete Carroll and Bruce Arians guessing as to whether Young Linebacker X is going to jell and understand what he's seeing. It's not totally under their control.
The best of them can try to fill outside straights, instead of trying to fill inside straights. :- )
It IS a little odd to me, though, that in 2016-17 the Mariners have gotten less pitching luck rather than more.