In terms of why it happened, I have two plausible theories:
1) The Yankees players are all clones of one another. Clearly they've been altered superficially to confuse us, but baseball stats don't lie and are apparently driven by gene patterns.
2) Summary statistics like wRC+ and OPS obscure variation in player hitting profiles, and all these players happened to perform similarly this year. Weird measures can occur randomly in small sub-samples of a sparsely sampled high-dimensional space (i.e. curse of dimensionality).
3) The curse of dimensionality has a sinister mastermind behind it, whose goal is to torture you through baseball oddities because they are impossible to explain. Or maybe the mastermind is God, whose goal is to bless you with baseball oddities that are too awesome to explain. Either way.
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