dimensions of crop marks

hello dear friends,

i make the follow steps in corel draw x8 :

1. i make a page 20x20 cm

2. i draw a square 20x20 cm 

3. the adove file ...publish to pdf (editing format with crop marks)

4.  finally import the above pdf file to a blank new page 20x20 cm.

The result is that the imported square is 20x20 cm but the crop marks ''cuts'' bigger dimensions from 20x20 (aproxx 20.16 x 20.16 cm).

Why?? 

Thank you in advance for your help

thanasis A.

Greece