There is no native resolution in VR in a sense we know for flat LCD. Even the display resolution of the headsets displays does not mean much in the whole scheme. The rendered image has to be rendered at the resolution (and most importantly the shape) which corresponds to the headset FOV, and then warped in order to cancel the lens distortion. The warping is not linear though so there is no straightforward relation between the rendered resolution and the screen resolution of the headset displays.
What the headset reports to OpenVR is recommended render target resolution which is what the headset designer believes is a best trade-off. This is what you get when you set the OpenVR supersampling to 100%.