This is just one of the unit related DC bugs. If you set SketchUp to not display your units format in ModelInfo>Units, then the whole house of cards collapses and all DCs revert to displaying Inches, whatever their internal unit is set to. It is annoying, as standard practice with metric units is that the unit string is always suppressed (neither do Imperial users spell out their Feet and Inches in their dimension strings). DC-s cannot even use the abbreviations (m,cm,mm).
Anssi