The window is based on the fastest nominal descent velocity. The time required to safe the staging interface, jettison the descent stage, ignite the ascent stage, and have the APS achieve a positive vertical velocity translates to 60 meters of altitude even if some of that is mitigated by APS thrust in the final few meters. If you're lower than 60 meters at a nominal descent rate, you'll hit bottom before all that has time to happen.
If your descent rate is slower, your window is higher.