[Mikou]

I'm going to assume that the font size and the line spacing and character spacing are the same in both files. (If they are different, that could certainly add to the difference in page numbers.) If they are the same: the pdf page breaks are set by the creator of the pdf. Many pdf readers are not reflowable, so they will simply shrink or enlarge that page to make it fit on the screen. The page, however, will have the same number of lines and will break in the same place no matter the screen size of your device.

Epub, on the other hand, uses a set page size. I'm not sure if it's the number of lines or number of characters that is used to determine that size. It creates page breaks where needed so as not to go over that page size limit. This is most likely what is increasing your page count.

This is the way I understand it. I invite anyone to add their thoughts if what I've said is inaccurate.