If you use Calibre, their "Get books" feature compares prices at a wide variety of on-line stores. I know that Publishers adjust prices regularly and their adjustments then appear in the on-line stores without any action on the part of the store. When there are different prices in the list price (which may or may not be the price being sold), that then impacts the price shown, which often has set a % discount (not talking about agency pricing).
I think that, at least some of the time, publishers try different price points at different outlets to see what happens.
Last edited by taming; 06-29-2011 at 08:28 AM.