I'm looking at the Toto Drake toilet to replace a 1966 toilet. The Specs for this toilet shows Water Surface 10-3/8" x 8-1/4". The dealer here has a Toto Drake connected so you can see the flush action. This toilet has no water surface. All of the water in the bowl after a flush is down in the opening of the trap. What does the 10-3/8" x 8_1/4" Water Surface Spec mean?