Sponges leaving out blocks

The bug

In a 7×1×7 space, a sponge placed in the middle is always going to leave out the west corners. Oddly enough, placing the sponge in a 9×1×7 (the east-west direction being the longer one) leaves out the exact spots as in the 7×1×7, but clears out the further water. This doesn't happen in 7×1×9 north-south, but it leaves out a 2×2 area of water again at the west corners.