I've just done this too. Didn't really want to have another piece of software running, but at least getting the correct reading on my rainmeter widget now.
I also have exactly the same problem. Freesync 120Hz monitor with AMD 6700XT. Games are running at a solid 120fps and this is also reported at 120fps by Radeon software, however HWInfo64 framerate reading goes up and down anywhere from 100 to 140fps.