What you can see on that page is that 1) 0.20% of Steam users had a Rift and another 0.20% had a Vive in January (and had it switched on and had launched the V

What you can see on that page is that 1) 0.20% of Steam users had a Rift and another 0.20% had a Vive in January (and had it switched on and had launched the VR software stack, I guess; I doubt Steam can tell a headset is present otherwise, so this is not a full count of VR headset owners); 2) for both Rift and Vive, that constituted an increase of 0.09 percentage points; in relative terms, that means the proportion of Steam users with VR headsets grew 0.09/(0.20 - 0.09) = 0.82, i.e. 82%. (You can also see that Microsoft Mixed Reality and Rift DK2 had a negligible number of users, so they don't affect that number significantly.)