Moving objects by arrow keys - Increment issue

Hi all,
Even thought I set the keyboard increment to 1 px, it moves 3-4 pixels at a hit on an arrow key. Setting it to 0.1 px doesn't make any difference. The distance of movent is not even. The first hit moves an object 4 px, the second 8 px.
I reinstalled the OS, the app, preference reset, ... nothing helped so far.
Any ideas?