It's not that they're just different varieties, they're from different combinations of prokaryotes producing different eukaryotic mycelium. You could take the same variety, inoculate with it's spores and produce two different looking mycelium mats. It has to do with the genders (for cubensis I believe there are 12) of the spores that produce prokartotic mycelium each, and come into contact, trade neuclei and become a single eukaryotic colony. Some pairs of genders won't interact and the rest will produce different consistencies of mycelium in different combinations.

The one on the left will produce significantly more fruit bodies.