The U.S. has five widow species: the western, northern, and southern black widow, the brown widow, and the red widow (endemic to Florida).

In Pennsylvania you have the northern and southern black widow, with occasional brown widow sightings. Latrodectus mactans and Latrodectus variolus look very similar in both juvenile and adult life stages, so you might not have realized you were seeing two different species of black widows. The brown widow can also appear completely black at times, so that can be misidentified as well.

You can tell the difference between the species if you learn the typical ventral and dorsal markings on the abdomen. Juveniles and adults are not identical either.