SteamVR... everything outside the circular target is aggressively culled with a stencil mesh and not rendered.
OVR does not [cull outside the target]] with the rationale that those "extra" pixels are potentially useful for asynchronous timewarp, which is expected to finally arrive for Rift with CV1.
If you don't render those pixels at all, you can't work that trick. Both approaches have their advantages in different circumstances. Culling makes it easier to hit your render target, timewarp/retroprojection makes it less disturbing for the user when you can't.