Eventually, you could come across that the Step&Repeat panel shows the same value in the Offset, Origin, and Size fields. This is due to Photoshop default units being set to Percent, which is actually a ratio and thus has no units. 

The panel needs a measurable magnitude to set positions and sizes properly and has been developed to support all other units that Photoshop provides, whether these are pixels, picas, points, inches, centimeters, or millimeters.