What I said is a statement, not a question. The question mark is not a mistake. What else could it indicate if not a question? Perhaps it was marking my general sense of confusion in response to the original comment. Or it could also be adding a hint of sarcasm. What would you like me say? Do you not approve of my use of it?

Throwamon's explanation is good too. When someone asks a question with an obvious affirmative answer, a popular answer is an inquisitive "yes?". Only the thing being questioned in that case isn't the answer, its the doubt in the mind of the speaker who asked.