Because ...
the holder for the skate consists of a fixed part (closest to the machine) and a moving part that you fasten the blade with. The grinding wheel is 6mm wide and located so that the inner edge is approximately 1 mm inside the fixed part of the skate holder.
Blade width varies between 2.5 to 4.5mm, and when the blade is clamped, centre on the blade is between 1.25 to 2.25 mm beyond the fixed part of the skate holder.

Measure the blade thickness an turn your dresser dial to the thickness of the blade before dressing the wheel. Then the centre of the radius (highest point of the grinding wheel) will be in the centre of the skate blade, not the grinding stone!