"The human brain has a low tolerance to hiccups in the VR space, and that's the spec we're confident putting our name on for a consistent gameplay experience when you're exploring our galaxy. Our spec also takes into account that we're not rendering at Vive's native resolution of 2160x1200 but at 3024x1680 (1.4x each dimension recommended by Valve) and then scaling down. We believe this is hugely important for anti-aliasing in VR to avoid noticing pixels so much, as other solutions can leave them appearing relatively large at the centre of your vision."Is your PC really VR-ready? • Eurogamer.net
That explains the recommendations.