"Because there is volume in there, crevice volume. In my engine design engineering class, we calculated compression ratio using stroke, crevice volume, clearence height, and combustion chamber volume, as well as any other variables needed to get volume. That crevice volume also contributes to unburnt fuel."
"Crevice volume" is not a component of compression ratio calculation. If you're describing notches in a piston, for instance, that'* a component. The "crevice" you describe is part of the cylinder volume and is not affected by ring height.
I think you're confusing "ring heighth: with "deck heighth." Deck heighth does make a difference.
Here'* a link that explains the process.