I first thought about the gradient that occurs from east to west across Chester County, as it can be significant. Have you contacted NCEI (formally NCDC) regarding this? Also, are there any available reports closer to Coatesville to better understand if those values are completely off the rails or not? As ChesoWX stated above, that location was a long standing COOP station. It would be interesting to see what the synoptic setup was during those dates, as I would imagine at least some included intense banding.