@Dave: that's not quite right, but I can't explain it well either. I'd ask Chem_Geek, but I don't think anyone would understand his explanation.

I'll try to get my son (bio-chem degree) to write an explanation that we can all understand.

@ Eric: I can't explain it, but I have verified it with a digital (and calibrated) pH meter. I should add, this ONLY works with distilled water; if you use tap water, the result will be closer to what you intuitively expected.