The reason is that I'm using a site called Replyz. Replyz searches through twitter looking for tweets that contain questions and then stores and displays them. So you can see questions being asked by people who you aren't following on twitter and post answers if you wish. The answers will show up in twitter as Replies.
If you wish, you can filter the questions if you only want to see questions in a particular category. I haven't used the filtering function which is why my answers may seem a bit random to my followers.
The "A:" at the beginning of my reply is just my own convention to hint that this is in answer to a question. Lately I've been trying to incorporate a simplified version of the question in my answer in case a follower might actually be interested in the question.
If you want to check it out go to http://replyz.com/ and request an invitation.
Update: You no longer need an invitation to use replyz.com