MusicXML - units conversions in exportxml.cpp

This is a question more than a bug (a question for Leon, I believe):
Why are the "tenths" units spatium-dependent? In other words: why is score->spatium() part of the formula for the variable named millimeters?

And fyi - I will be dramatically simplifying the unit conversion formulas in the very near future. If there is anything I should know about them that I might not be able to figure out on my own, please inform me.