in arenas it does that because back then you started at 1500 and they never bothered to change it I guess
not sure about rbgs, they might have just copied the script; but in the end, even if it's a bug, it might never get fixed since they don't care enough about pvp
the reason for <1500 achievements is that they probably wanted to have rbg peak ratings be about the same as arena ratings, and since they needed to put all the old pvp titles in they had to start low. besides, the amount of bgs it takes you to get to 1200 or w/e the first achievement is, is about the amount of bgs it took you to get private back in the day