Only one row at a time has voltage applied to it. In one update, the image is scanned out multiple times, so it appears as if all pixels were changing simultaneously (and perhaps they do, if the electrodes have significant capacitance.)

With fewer rows to update, each row gets a push more often, flipping the grains faster.

Hypothesizing a bit here: maybe it's like flash, where erasing is slow and has to be done in large chunks, but once a large chunk is erased small chunks can be written (once) quickly.