That's probably related to
https://github.com/koreader/koreader...ts.lua#L50-L51 (the intent being that you keep 30px of the previous screen visible, for clarity in general, and in particular if the bottom of the screen happens to cut through a line
).
You can tweak that as part of the "Advanced settings" UI. ([Tools] > More tools > Advanced setting).
What you're asking should be doable by using a value closer or equal to 0.
Fair warning: continuous mode is a giant can of worms, expect stuff to break in fun and interesting ways depending on zoom modes & factors and whatnot (and I'm not actually sure anybody ever validated what happens at 0).