It's not that surprising. The gap is very small because the player doesn't fall until they're completely off the platform. This is when the player starts to fall (when moving slowly, moving faster it would go even further off the edge before it starts to fall):
<img src="https://dl.dropbox.com/u/8367729/construct/pics/walkovergap.png" border="0" />
So the gap to cross is less than 32 pixels.
Depending on the movement speed and value of gravity, you can make it across the gap before you fall down.