E-ink displays can refresh faster
If you hold down left / right on a Sony reader, it pages through content fairly fast. I assume it does this by skipping some expensive page "reset" action. This wouldn't work too well over time because the display is not pixel addressable in the same way as an LCD so ultimately it would leave a smudge of e-ink capsules stuck in the wrong state. Hence the need to do a period reset to fix the state of the display.
Maybe Bookeen have figured a way to reset the bare minimum number of capsules and to only do it based on some quality threshold. The display would appear to be more responsive, but at the cost of some minor degradation until the refresh kicked in.