I am not exactly sure about this answer, but it is what I have read here on the forums. Please correct me if I am mistaken.
Your driving average is calculated based on all of your drives starting from the day you began. It also only takes into account drives hit using your driver, nothing else. I am not sure if drives are counted if they end up anywhere off of the fairway, I know that in order for a long drive to count in your stats it must land on the fairway or the green.
I have also been averaging 280ish yards per round and have slowly watched my average creep up from a 268 to a 270. It is frustrating but I have to take into account some of my not so steller outings where my average was a 260 or so. It goes straight to the bottom line.
Just keep up the good driving averages and eventually you will start to see your average go up, but don't expect it to shoot up rapidly over night. It will all start to balance out.