You called Reservations with a Revenue Management question. You will not get a useful answer. Of course the generic answer is that fares go UP and DOWN all the time.
As for the partial refund, I vaguely recall hearing something about that here a while back, but I could be hallucinating. Consider yourself lucky. Alternatively, consider flying Jetblue which always gives you a credit voucher (for the full amount) if the fare drops. I assume you can do the same on Southwest.