But it isn't neutral average language; it's extremely idiosyncratic. GPT-4o used present participles something like 5 times more often than the average human, for instance.

Though there evidently was an attempt to make GPT-5 sound more neutral, but in a literal sense: the emotional content of its text was flattened out. Suddenly it wasn't nearly as sycophantic or enthusiastic, and wrote much more dry, neutral text.

I think it's more accurate to say the text is not regressing to a mean, but to an artificially constructed style defined by the training process. And the AI companies have only partial control over that style, so weird things emerge from it.