The default interpolation of the vtkLookupTable is curved which is not standard in medical applications.
In mitkImageVtkMapper2D.cpp in line 757 a default VTK LuT is created as default property. As for the member of the localStorage (m_LookupTable) the interpolation should be set to linear. (e.g. vtkLut->SetRampToLinear();
Very easy to fix.