Points aren't being constrained to the grid when using Snap to Grid in Xara.

If you zoom in using the zoom tool you will see that the snap to grid is behaving perfectly.

Viewing the grid settings shows that they are by default set to '50 pix' for grid spacing and '50' for the number of subdivisions. This means the snapping is set to snap to whole pixels. This is a common and useful requirement for web screen work. Snapping your objects to fractions of a pixel can generate anti-alaised edges.

If you set the number of subdivisions to '5', then it will snap to 10 pixel boundaries, which is more visible at 100% view.

To configure the grid options select Utilities followed by Options and click on the Grid and Ruler tab.