plane fitting problem with GeomPlate_BuildAveragePlane

I want to make a plane which fit scattered 3D points. I tested 'GeomPlate_BuildAveragePlane' algorithm with analytic plane data. Tested points are equally spaced in x,y domain with same z(ex. 300.0). But z value of result plane center is different with input z value.
I set 'POption' to 1 and 'NOption' to 1.
I can't find any problem in this simple test. Please I need your hand.