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First I couldn't visualise your explanation, but after researching I've seen a few examples and finally got it.
I've made it extrude 100mm and 100mm it consumed, so it's not the extruded so it must be the X and Y axis.
I've tightened the belts but the error didn't changed when printing a calibration print of a small cube.
Now I see that I have indeed to alter the DEFAULT_AXIS_STEPS_PER_UNIT variable in the firmware.
Shouldn't this be "perfected" when the maker produces a new version of the firmeware, or is this something specific from each machine and the user should tune it (if I made myself clear)?