I've noticed that ChatGPT (whatever model the free version uses by default) likes to phrase answers as though it's correcting me, even when my question doesn't contain any assumptions.
I assumed this is a system prompt or RL that nudges it to be always skeptical but then it still has like all the models the urge to appease the user.
The "It's not X, it's Y"-style repetition is another example of that: It argues with itself in the background, and the argument leaks out as if in refutation of things that you (the human) never had imparted into discussion at all.
That can be annoying, but it also happens in debates between people, and sometimes the implicit assumptions are real and (sorry) load-bearing. So it can be an appropriate conversational tack.