Yeah, you're right, something is definitely off there. I just went back and rewatched Digital Foundry's RX 480 benchmarks, and they're definietely higher than this charts suggests. The charts says 52.6 fps for The Division, DF says 54.7 with the same settings. The charts says 47.9 fps for Witcher 3, DF says 61.2. The chart says 41.5 fps for RotTR, DF says 61.1 (different AA method though). The chart seems to be using AA methods that favor Nvidia cards. But, there is one oddity there, Witcher 3, there is only 1 in-game AA method available, yet there is a massive 13 fps difference between the chart and Digital Foundry testing. I'm not sure what the culprit is here, maybe Nvidia turned on their Game Works features on RX 480 even though they're not meant to be ran on AMD hardware?

I'm not sure the difference between the 4gb rx480 and the 8gb rx480 would really have an impact in terms of that comparison chart, having more vram doesn't generally give a boost to fps afaik, it just lets you do more STUFF (bigger textures etc).

The 480 4GB also have slower memory, rated at 7Gbps instead of the 8Gbps of their big brother. That limits the bandwidth of the card and affects the performance in some games, as this article shows: http://www.gamersnexus.net/guides/2503-amd-rx-480-4gb-vs-8gb-benchmark-is-it-worth-it

I'm not sure the difference between the 4gb rx480 and the 8gb rx480 would really have an impact in terms of that comparison chart, having more vram doesn't generally give a boost to fps afaik, it just lets you do more STUFF (bigger textures etc).

If you fill up the VRAM it can drastically reduce performance. This happened with the 2Gb 960 in some games iirc.