Device ID in Facebook: Mobile Safari vs Facebook app

I have an iPhone 7 Plus running iOS 11. Whenever I log into Facebook via Mobile Safari or Firefox using my iPhone, Facebook's log in settings indicate I logged on with an iPad. Conversely, if I log on FB via Mobile Safari or Firefox using my iPad, FB's log in settings indicate I logged on with an iPhone. If I log on with either device via the FB app, each device is correctly identified. Does anyone have an explanation for this behavior? I contacted FB over a month ago and, not surprisingly, haven't received a response. The same behavior was also present in iOS 10 and perhaps earlier.