I also notice things such as when they stop the $400 OBC bonus on one sale that the regular price drops by about $400 on the next sale and vice versa! There is always a sale. There are some differences, but they seem to be minor. As someone stated above, they pretty much seem to stay about the same.
However, I'm thinking they must have some sort of algorithm tied to the price structure on the website. If the ship is not booked up very much, as in the case someone mentioned in a "fire sale" earlier, the price drops. As the ship gets close to booking full capacity early, then the price jumps dramatically.