It's barely rating the quality of the actual water coming from the taps. That's a single datapoint among 6 other factors, like customer service of the water utility.
And the source of the ranking (JD Power) doesn't really list any breakdowns of that either.
Yeah as an environmental chemist I was sorely disappointed. To be fair it would be grueling to compile all that data though. I don't know where my water quality report is hosted and I'm probably the one who analyzed it!