I think the reason is because semolina isn't ground as finely. The more finely you grind a flour, the less it weighs per cup, cake flour being a good example. (Powdered sugar is another good example, it is much lighter per cup than granulated sugar.)
A cup of durum wheat berries is 192 grams, other hard wheat berries are about the same, soft wheat berries are a bit lighter, 168 grams per cup.