Of course that's not the only valid comparison point. Who's getting tested is absolutely a function of number of available tests. If there are tons, you can test more, and will test more borderline cases. You could have two places with equal infection rates, but one is only testing everyone randomly, or testing borderline cases, because they have excess testing capacity and the other is only testing people with severe symptoms. Place 2 will have a much higher positive test rate, but not because they're worse off. They may well become worse off, if they're not catching mildly symptomatic spreaders, but something that can easily be manipulated by a variable that isn't infection rate just can't be your only metric.