Congratulations on the price drop!!!
Cruise pricing can be a mystery but it is based on supply and demand. Certain categories of cabins are more in demand than others. Low category insides and low category balconies are the most in demand because the cruise line can sell them with good lead in pricing. "Seven night cruise from $499".
Almost every ship that sails is full so they do whatever they can to fill it. If the cruise is selling well then the price won't drop and could go up. If they need to sell it out then you get a price drop. How often this happens is anyone's guess. Enjoy the rebated money and if you get more, great. If not then enjoy!!!!