A lot of scientific studies, there is some run over but the maps show the heavy concentrations of the insects. all data is compiled from different places and then put into maps.
As for yellow sallys there are several different species and sizes to found in pa. here is a pic of a size 12 medium and a size 16 small yellow sally, 2 different species of stone fly both from Pa.
Sand, you should post a close-up of that nymph so we can see some detail. May be more appropriate in fly tying forum. From what I can see it looks great.
When I see a distribution map like that, I question them too. Spotty distribution over a wide variety of terrain. It just reeks of missing data. I'd guess that the species depicted in that map could be in every single county in PA.
Cept maybe Philadelphia, because the only animals that live there are rats and city slickers. j/k.