Both Nikon and Sigma design some of their lenses to focus beyond infinity.
There is an explanation why on this site. Read the text to the right in the gray area.
All my Sigma lenses has this.
The effect is different on different lenses.
On my Sigma 8 I can not see any difference even if I focus beyond infinity.
But my new Sigma 10-20mm is very sensitive.
Below is a crop from 2 images taken at 20mm / f5.6
|Focus set at the infinity mark||Focus turned all the way beyond the infinity mark|