Tap water doesn't have a SG of 1.000, only pure water does. There are enough ions in tap water to keep osmosic activity to a minimum. The more pure the water, the more chance of engorging your yeast cells with H2O you have. Grab yourself a microscope and some DI water and see for yourself.
Here is a video of hemoglobin in distilled water:
(I know they aren't structurally the same, but the principle applies)