Table column width in xml changes from mm to * after save

Hi DITA users, I am trying to understand why FM changes the colwidth units of my tables from mm to * each time I save the xml files. Any ideas?

Example of a table with three columns.

before save:

<colspec colnum="1" colname="1" colwidth="35mm" colsep="0"/>
<colspec colnum="2" colname="2" colwidth="25mm" colsep="0"/>
<colspec colnum="3" colname="3" colwidth="100mm" colsep="0"/>

after any change in the xml file and then save (note that I did not change the colwidth, they got changed autonomously): 

<colspec colnum="1" colname="1" colwidth="22*" colsep="0"/>
<colspec colnum="2" colname="2" colwidth="16*" colsep="0"/>
<colspec colnum="3" colname="3" colwidth="63*" colsep="0"/>

consequently the width of each column and the whole width of the table are different after each save.

Now if I change the units to mm again and save they stay in mm until the next time I change and save the file.