My pdoc usually has his arms crossed. This can mean someone is closed off or distancing himself. Or, if he's done courses in NLP (neurolinguistic programming), purposely trying to portray that (sort of a silent Back Off!). As he's very kind and engaging, I think it just means he finds it comfortable to sit like that.
I rarely look at my T so I have no idea how she sits.

A few years ago after a test at school the surveilling teacher (the teacher there to make sure we don't cheat) approached me the next day asking if I had been looking at my neighbour's paper (and thus cheating). I told him the truth: I have problems with my neck and had a really severe pain and I couldn't keep any normal position - or any position at all for longer than two minutes.

There can be a lot of reasons for what someone is doing. I wouldn't worry about it unless it happens a lot. If you want to bring it up discreetly, you can ask "Are you OK? You don't usually sit like that."