Technically, they weren't. The FRT table doesn't have force of law, so even if a rifle is marked Prohib in the FRT, it can still be NR. It's definitely the case with this one.
A rifle could also not exist at all in the FRT table and be NR. As long as the rifle satisfies the conditions to be NR as they are written in the firearms act (barrel >18.5'', etc...), it is NR, no matter what's in the FRT. To explain it in other words, it's the job of the FRT to conform to reality, not the job of reality to conforms to the FRT.