Precision issues
I observed that we can only apply the restriction and prolongation operations a certain number of times until the projected constant values in my test are not equal anymore to the reference values (with respect to a tolerance of 1e-12).
For more infos see the todos in 2D and 3D definitions of GenericEulerKernelTest::testFaceUnknownsProjection(), and GenericEulerKernelTest::testVolumeUnknownsProjection()