I don't find it suspect (well, suspect of what exactly?) as a native to these cities but I may lack perspective. Go one city over all over the Bay Area and you get wild differences. There's a lot of land between north bay cities too. Also, weren't we all just using these divergences a year ago to back up Covid statistics - i.e. Santa Rosa was higher than surrounding cities from my recollection, and that was "expected" one you knew the areas.
an eighty year old in either place may rarely talk to anyone, while a commuting Dad with kids in pre-school may get involuntarily exposed every single week. There is some truth to geographic locality for disease, but there is also Car Culture in the USA.
This is not suspicious at all. San Rafael has a combined sanitary and storm sewer system. In the California rain season their sewer is diluted with rain water. Novato has separate sanitary and storm water systems.