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James is capturing, among other things, big increases in playing time.
When a player gets a big increase in PT in Y+1, James is positing that he's less likely to make a big stride in Y+2, than if he'd been playing a lot before that, like Jose Lopez has, for instance. Otherwise, you're asking for two consecutive big leaps (in absolute value) in two years.
Whatever the skepticism towards R and RBI, a guy who goes from 50-50 to 105-105 has consolidated some things -- even if it's only the ability to withstand "exposure," to hit all pitchers. Usually a plateau leap comes after a period of stagnation/consolidation.
That seems very sensible to me, and as a completely separate issue, it WORKED well.