Comment by Analemma_

Phones and other devices with cellular radios still use differential GPS by way of the cell towers (which have known, fixed positions) to do those corrections. You can notice this because, when you don't have a cell signal, your phone is considerably slower to get a GPS lock and less accurate when it does. Presumably these farms are out in the middle of nowhere and so that's why they can't do the same thing and need a satellite-based augmentation instead.