Onlyfans changed things a couple years ago so all vids get re-encoded to be the same, most likely to ensure compatibility across all devices, sites like youtube do this for the same reason. Onlyfans made it so _source aka highest resolution is 23.98fps with a bit rate of around 6000kbps and _720p is 50fps with a bit rate also around 6000kbps. I dont think anyone knows why Onlyfans chose 50fps for 720p, it results in no file size savings over _source, seems like a mistake they never fixed tbh.
Before the change framerate etc was probably just however creators uploaded vids.
The numbers have changed again recently with DRM vids which are at 29.97fps (basically 30) and 8-9000kbps for _source, no idea for _720p DRM vids because the scripts required default to _source.