Virtual Texture Scale does not work well

In Oculus Unity Integration Guide, Virtual Texture Scale is explained as "dynamically scale the cameras render target to values lower then the ideal pixel fidelity, which can help reduce GPU usage at run-time if necessary."

I supposed I could simply lower the scale, but when I modify it at run-time, zooming effect into the left-bottom of the screen happens.

Here are the screenshots. I feel this is a bug. Any ideas?
VirtualTextureScale.jpg