Lane It's probably because the hardcover tends to be larger type on thicker paper, whereas an ebook they're compartmentalizing everything. Thinner margins, less space between the lines, smaller writing (unless you have the size huge).
Shiah I have a nook but its a nook color and sometimes it has fewer pages because of the size. Sometime different copies of books do have missing chapters and stuff but not to often. If its on an e-reader then its probably the size of the pages.