My printed size is smaller than actual size!?

I make a 5x5 cm square. It looks good on 100% preview. (I used ruler to measure on the computer.)

However, If I print it, it shrinks slightly, appx 96%. I have made sure that the printing setting is "Do not scale". 

Then, I have tried to export it into a pdf file, and check with the Adobe acrobat using actual size preview. The square is smaller too on the computer.

I can never get a 5x5cm square on the paper...

Please help! Did I miss something?