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Yes I am sure. The error can be measured in the STL file, which means the error has nothing to do with the slicer or printer. If you open the STL in MeshLab and use the measure tool you can see the error clearly.The pockets vary in size.

It is also visible by the naked eye directly in OpenSCAD now when I am aware of the error. Make the brick 2x2, press F5 and view the bottom side with CTRL+5.