Physx Character Controller 's Shape always hover from the ground

I tried to use Physx to impledent a Character Controller(PxController). Physx supports two Controller shapes: Box and Capule.
They are defined by PxControllerDesc.

Even I set the description’s contactOffset to a number that is very close to 0.

Both of them showed an unexpected result:
The ground was on the plane whose high was 0.

I wondered if the character was actually hovered from the ground. So I used getFootPosition() to get the foot position. The result is unexpectedly 0. But from the image the result should be a result high than 0.

Why did the physx show that strange result?
And how to solve this problem?

See also: