Part assembly and drawings can be set to different units of measure.
Its under tools - options ducument properties - Units
I do not know the exact reason, but it displayed the same for me too.
To further test it, i selected another sheet size ( & not selecting "OK"), it changed to inches. Reselect the required sheet size, now it shows up in inches.
Probably, the default unit set/programmed was "mm". Funny thing, but surely misleading.