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My guess is that Pokemon with Contrary have tissue and muscles rapidly improve tension or some shit everything it's stat gets lowered by more "physical" ways.
Like if Serperior uses Leaf Storm, it's muscles that fire off its projectiles fight back the stat decrease by making it stronger.
And when it uses growth, the muscles are tricked into doing the contrary.
While with moves like Swagger and Screech, Serperior mind would just do the contrarian thing to do.
