Illustrator :: Cannot Align Any Rectangle Or Define Specific Size

i've some trouble with a specific document. I could not align any rectangle or define a specific size. For exemple, i choose 600px x600 px in the top "menu" bar, illustrator change the values by 599,641 px

It's the same in inch, mm..etc.. The second thing it's if i choose a specific coordinate to place the rectangle , it's change by illustrator.

I have not the problem on other document.

It's a 21 cm 21cm @ 300 dpi, proprotions 1

No grid activited or anything else to force the magnetism