I believe the technical term for the type of recovery used by photorec (and I am assuming the other) is "file carving".

As far as I understand it, it works by reading the disk blocks one at a time, attempting to match the start to the blocks with "signatures", or patterns to identify known file formats.

It can only recover file formats it has signatures for. Also, the recovery program needs to be able to supply an file extension to apply to the recovered file for the user.