The model is fined tuned for chat behavior. So the style might be due to - Fine tuning - More Stylised text in the corpus, english evolved a lot in the last century.

Diverged as well as standardized. I did some research into "out of pocket" and how it differs in meaning in UK-English (paying from one's own funds) and American-English (uncontactable) and I recall 1908 being the current thought as to when the divergence happened: 1908 short story by O. Henry titled "Buried Treasure."