Prices can go down as well as up. All airlines use a very sophisticated computer programme which manages the revenue from each seat. The object is to get as much revenue or "Yield" from every seat. Depending how fast or slow seats are selling,or what day of the week, time of the year all will have a input as to what the airline will charge. For example Monday morning flights to LHR will only have a few cheap seats on offer, but will have plenty of high fares because this is a very popular time for business people. Alternatively Mid day flights will be the opposite, with plenty of cheaper fares available.
Unfortunately you booked just before the computer had reduced the prices as seats were not selling fast enough at the higher prices.
Usually the cheaper seats sell out fast and early, but not always. There is no question of ripping people off, although one can feel like that in a situation like yours.