I see this with myself with code reviews. I tend to nitpick more on good reviews, not because the (minor) problems seem "relatively" bad, but because I have the energy to report them (and I assume the reviewee has the energy to fix them). Whereas on very bad reviews, I can let a lot slide, because I spend so much energy convincing the reviewee of the most egregious issues with their code.
Same deal with "neighborhood watch": the watchers probably have an idea of the "ideal" neighborhood in their mind. At first, they report only the egregious problems, because doing so takes energy both on their part and on the part of the police. Once those problems are "cleared away" they're free to focus on the more minor issues.
(Though I suspect that my explanation, and the explanation offered in the article, are actually two sides of the same coin.)