I think this is just the nature of LLMs as predictive generators.

The model is predicting the type/level of conversation based on what the other party is saying.

The most typical types of conversation are of two peers, so by default the LLM is likely to respond to you at your own level, unless you ask it to behave differently.

As always, prediction goes deep. The best response to Terrance Tao is Tao-level math.

It reminds me of reading how LLMs continue chess games if given a partial game - they have learnt to assess player strength based on the moves they make, and will predict game continuations based on the perceived strength of each player, predicting (generating) poor quality moves for a weaker player. This isn't an AI playing chess to win - it's an expert predictor predicting what comes next.