Good deal! Really glad I learned this now....rather than later. I was positive that the grommet was 'made' to fit over the o-ring....it sure as heck looks like it should be this way - apparently not though.

I can see now, clearly, in the picture you posted how the tube appears to bottom-out, when inserted into the drain hole, and uses the o-ring to make that sealed connection...which means the grommet HAS to be a little higher up...forming a second seal on the top side of the drain hole.