Because it's considered a weight-neutral med. Doesn't mean can't, but it'd be unusual. Insert here the usual disclaimer about meds not affecting everyone the same way etc.
What occurs to me is that muscle weighs more than fat. Between all your exercise and saying "technically" overweight... it certainly seems possible, especially when you're talking 12 lbs and over the span of a whole year (as opposed to if it were, say, 60, the muscle explanation would be quite unlikely(!)) And by "technically", do you mean that it doesn't look it, but the scales say more? Being in exercise, you probably already know that conversion to muscle would not be a bad thing at all, and certainly trumps the number on the scale. Worth considering the theory, I hope.