Photoshop :: Why Ps Keeps Changing Settings For Placing Objects / Image Size

why Photoshop keeps changing settings for placing objects, image size etc.?
 
For example: I create a new Photoshop document:
 
     ppi: 300
     Width: 160 mm
     Heigth: 160 mm
 
after clicking "OK", these go to 160,2 mm or less than 160 mm.
 
The same with the sizes and places of objects etc. I create a 20 x 20 mm circle. One click later it is 19,89 x 19,89. I set it's location to X: 80 mm, Y: 80 mm. One click later: 80,02mm