It pulls counties from OSM administrative boundaries of level 6, which according to the OSM wiki, is "State counties and county equivalents, Territorial municipalities" (https://wiki.openstreetmap.org/wiki/Tag%3aboundary=administr...). It does lead to some weird oddities, like counties with under 10 homes... I'd rather not manually correct it, since I want to rely on pure OSM data. (Unless you mean there is an actual bug in the processing and there's counties listed that aren't in the right states...)

I'll add a link to the OSM relation for the county to each county page, so you can see the source data on OSM to verify/edit.

There are also not 53 states.