Personally, I think that the counting-by-tens thing is weird, too, but then again, I’ve seen pinball games where the last three zeros of the score were just painted on. People like big numbers. People are weird.
And really, we haven’t seen enough Quidditch games in the books yet to say what a “typical” Quidditch game is like. Maybe most games see a few dozen Quaffle goals before the Snitch is even spotted, in which case a 15:1 ratio might not be unreasonable.