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:
http://uploads.gamedev.net/monthly_10_2015/post-224872-0-42161000-1446133013.jpg
http://uploads.gamedev.net/monthly_10_2015/post-224872-0-84647200-1446133000.jpg
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:http://www.gamedev.net/topic/672848-physx-character-controller-s-shape-always-hover-from-the-ground/