When you set the map height to 22, remove all blocks in row 1-23, put some creeper in the map and hit auto advance, the creeper also moves in row 23.
In this pic I put some creeper on the right side of the wall and reversed gravity a bit later.
This is something different than what you think. If you remove all terrain in the editor, then shrink the size, and place creeper, it will fall through. I believe I posted about this a long while back. :P
right, WAD, virgil doesen't remove the lower rows from the flow algorithims