Perhaps to provide contrast between correctly followed and incorrectly followed election procedures. I am not certain. Mastriano also said he had requests from counties for a forensic audit that are not included in the original list of three counties. I assume those requests are coming from Republican counties if that is the case. My last count of Republican vs. Democrat counties in PA is 56 to 11. It is possible that there is fraud in Republican counties. My wish list of the first round of counties would have included Montgomery County. There have been indications that there is more fraud there than in Philadelphia county. Philadelphia county votes 80+% Democrat. There is actually less a "need" for fraud there. If one was to target a county ripe for fraud, they would choose a county that is Democratic, but with a larger number of Republican votes to flip.
Yes . . . Densely populated, single-party control, and many election 'irregularities' going back years. Home county of AG Josh Shapiro, coincidentally.