If customers report a lower price elsewhere, Amazon will often lower theirs so as not to be undercut. This is true for everything they sell -- to the extent that they can reduce the price.
For ebooks, I think Amazon will almost always be the lowest price because of this. And there are a lot of ebooks that are exclusive to Amazon as well.
There are also some of the big publishers whose agreements with Amazon, and presumably other ebook resellers, that restrict how much the reseller can discount the ebook.