Why absolutely flat surface looks deformed at normal map texture?

Hello. I have pure flat highpoly geometry (i checked normals, scale and vertiсes position along certain axys)
Here is highpoly render

Here is also geometry with no SubSurf modifier.

But when i bake normals via xNormal i recieve strangely deformed map.
Here how it looks at normal map.

I do not understand why this thing happen? How prefectly flat geometry become weird and bake as bad normal map?

I find that using creases causes those sort of deformities where uncreased edges are.

Thank you, Anthony Forwood I was surprised but problem was not in Highpoly - it is perfectly flat, indeed. Problem was in Lowpoly and weird shading of sharp edges in combination with ShadeSmooth display mode. AutoSmooth and Mark Sharp solved the problem.

It is also a pleasant surprise that Blender can export information about sharp edges in *.obj format and, more importantly, xNormal can recognize this sharp edges and bake normals taking this info into account.

Here is how this normal map looks

perfectly flat, as it must be.

Thank YOU. You just taught me something. :slight_smile:

Just to add,
tutorial and info about one of the latest improvements: