ZBrushCentral

Displacement differences in maya and zbrush

Hi,

I would like to make a window, but the maya’s render is different from the zbrush at the corners.
Here is the pictures:
Window in zb and maya

I followed this tutorial, using zb3.1 and its multi displacement 3:
http://www.zbrushcentral.com/showthread.php?t=43264

Could anybody tell me why the differences are at the corners?
thank you