Per this post, be aware that there is a bug in the metric structural beam family for W shape beams under 200mm in depth. The developers are looking into the problem and I will report back on their findings. This affects the M_W-Wide Flange.rfa family that ships with Revit Building 8.1, 9.0 and Revit Structure.

These beams will sit lower (37mm in one case) than their reported height. As a workaround use the Imperial W shape family and change the values to those needed by the metric beam.