Supply and Demand could be it, but I think it because amazon have multiple vendors selling. Since the lowest price is almost always taken the vendors jockey their prices to beat out the other vendors on the site. I looked into selling on amazon last year and found it to be very difficult and decided against it.
True, that also has a significant impact though it's not always the lowest priced displayed. In fact, if you click on the link with more offers, you can some times find a cheaper price. Not always, but I have found that to be true quite a few times...plus, I've also seen Amazon's own price adjust throughout the day. It's definitely not a straight forward simple formula that is used, but it is designed to make sure the price meets the demand at that particular time.
Edited by tokendog - 11/26/12 at 9:08am