Depends on the aspect ratio of what you're asking to "fit" to. If it's (it being page or content, depending on your settings) a perfect square, fit to width == fit to height, so a page turn will turn by a screenful of content, which will also mean a page of "it"
.
But that will probably never happen, because mostly, you'll be reading content that's Portrait-ish; i.e., taller than it is wide. So, if you fit to width, you're left with a bit of "it" offscreen at the bottom, so a page turn will let you actually *see* that leftover instead of skipping over it.
The vertical movement is always worth (roughly) a screenful, whatever the screen currently displays, so whether or not you get overlap actually depends on the content (and how much of the content you get to see in a single screen depends on your settings).
As a mildly related example, for a CBZ use-case, on a large screen, I target a 1:1 experience, so I preprocess my content to target the *exact* dimensions of my screen, and then fit to page height (or 1:1, it'll be the same anyway).