The ePub format presents page numbers against a standard page length, so that the number remains consistent across all devices, regardless of screen size and the font size you are using.
This means that you will page forward through several screens before the page number increments (if the amount of text on your screen is less than an ePub standard page).
If you search these forums you'll find a lot of discussion of the benefits and disadvantages of this. I happen to like it as it gives me a consistent measure of how much is left to read, in a way that screen page numbers or percentage bars cannot do. However, many people dislike it.