Hardware Canucks appear to be taking their results from an open test bench
RX 5700 being 48 dB, so maybe something else is happening there, like scaryjim suggested. Maybe HWCanucks have a very intensive test, I could just see in the corner of the monitor, the graphics going crazy in video. Had a quick look at AT again and they're testing with a NZXT Phantom 630 Windowed Edition case, but doesn't say if the side is off or not, but that'll explain it probably.
Edit: Although the Hexus review was apparently not open bench test it was using a "be quiet! Dark Base 700" case? .. and Hexus pulled off 39 dB? Anandtech 50 dB within a case and HWCanucks got 48.7 dB open bench test for the 5700? Hmm..