A matched ground set is only needed when the bearings are mounted without a spacer between them, or a spacer on the inner AND outer rings. You would need to order them with the preload you want, light, medium, heavy, etc.
The way these mills are designed, the bearings are spaced apart to increase the rigidity of the spindle, it also helps with heat dissipation. The spacer provides the spacing between the bearings on the outer ring, as well as keeping them parallel. You can adjust how much preload you want by how tight you make the nut against the top bearing inner ring. If there was a spacer on the inner ring as well, it would either have to be precision ground to the amount of preload you wanted for the bearing set, and then you would just tighten it down until it stops and not worry about adjustment, or you would have both spacers the same height but use bearings with the appropriate preload ground into the rings. It’s a lot cheaper to do it with a single spacer and manually adjust the correct preload.