It can also mean that they aren't very good at making conversation

In which case, why would I want to make conversation with them? :-)

or even that they're not sure if you're just being polite, and aren't interested in a longer answer. I often refrain from answering people's questions in detail if I think they're just asking to be polite.

Good point, though there's a middle ground between answering with as few syllables as grammatically possible (what I usually do when I wish someone wasn't talking to me in the first place, but I don't want to be excessively impolite) and a long answer mentioning personal feelings and asking a question back.