I got 100 surfaces split vom one double curved surface, which curvature is not equal in any direction. If I measure the curvature, for example the minimum and maximum principal curvature. How is it possible, that the curvature is always the same? There are obviously some surfaces, which are more curved, than others.

the term trimming means in Rhino to “mask” a surface.
All of your surfaces are identical but have a different trim/mask. So whenever you say, parameter at .5,.5, you get the same values for each subsurface. In order to get different surfaces, untrimmed, you can use split-iso-shrink(rh) or isotrim(gh) . Or you get the midpoint of each surface and use surface closest point to get the local parameter. hope this helps