In the case of third-party sellers, it apparently sometimes happens that two sellers will have automated pricing systems that try to determine what the market will bear and set the price accordingly, and they get into a kind of "arms race" and each one keeps escalating the price to be just a little above the other. A couple of years ago this process went berserk and the results made the news, but it's possible that similar effects sometimes boost prices unreasonably without getting completely ridiculous.

-NT