I’ve explained this here:
WMR Scaling and Dev Tools - Some Explanations
The difference between the G1 and G2 is due to the curvature of the lens most likely.
PS: the article is old too (relatively) because when I had the G1 in June I’m nearly certain 100% SteamVR SS was very close to the native panel resolution, whereas the article is saying at 100% it is about a half of it only.