Try to do this:
1) Create a document (a) with mm set.
2) Create a document (b) pixels with px.
3) Draw squares in document a. You'll notice that the width, height and coordinates show you px, not mm. This happened since you have opened a document with pixels (document b) in step 2).
The remedy to the problem: open "Document Setup" and just hit OK.
Voilà: we have mm.
4) Switch to document b. Draw a square. – Yikes! It's showing mm for width/height/coords.
Solve the problem by opening the "Document Setup" *again* and hit ok. Now you got px.