Are <system-distance> elements too small in exported MusicXML files?

Is it true that the values of system-distance elements output by MuseScore 2.0 are decreased by twice the "spatium"? (musescore-dfabf91) This would mean that those values do not really correspond to what the MusicXML documentation says:

"The system distance is measured from the bottom line of the previous system to the top line of the current system."

In the source code I found the following lines, which may confirm my suspicion:

Expression for the system-distance in ExportMusicXml::write, line 4128:
m->pagePos().y() - previousMeasure->pagePos().y() - previousMeasure->bbox().height()

Setting of previousMeasure->bbox in System::layout2, line 365:
m->bbox().setRect(0.0, -_spatium, m->width(), systemHeight + 2 * _spatium);

The addition of 2 * _spatium here seems to make the bounding box of a measure too big, and therfore
the system-distance too small. I also found that Finale Notepad behaves in precisely the same way (i.e. system-distances are too small). I could confirm the seemingly erroneous values of system-distance by measuring pixel-distances in a .png file corresponding to an .xml file

Could somebody help me understand this seeming mismatch?

Willem Vree