Seems like both of you overly complicated this particular issue (maybe you are more alike than you realize). You were focused on a minor issue of sitting/standing which really wasn't about your therapist at all. He was focused on a minor issue of not disclosing too much when a simply "I have a bad back" would have sufficed. It seems like you tend to get stuck on the details and he kind of has that same issue with details in that he is trying too hard to keep that distance through not disclosing details.

The happy medium would be 1) you stop getting hung up on the details and just accept "no" as an answer without continually harping on the details once someone has told you "no" so that you can stay focused on the bigger picture, and 2) your T stop getting so hung up on not disclosing details that it becomes a bit ridiculous and just feeds your drive to know more detail.

You said: "There was also something in there (later in session) about why he doesn't disclose more, and he said about how my mind is 'a steel trap,' which is the same as a phrase my mother has used with me. And he said how I hold onto to any little piece of information, which is the sort of thing she's said, too." This struck me because I think what your T is saying is that you do tend to get stuck on those little details and its like you can't let them go. He is sort of walking a tightrope with you. If he gives too much information, you'll over-interpret all those details and start focusing on him instead of the work you really need to do in therapy, and if he doesn't disclose enough, you pretty much do the same thing -- you over-interpret any lack of information as having meaning and get hung up on it. He's sort of damned if he does and damned if he doesn't.