The article indicates the observations were for a "equilibrium zone" of the Greenland ice sheet. As the ice sheet changes, I would also expect that the location and size of a zone thusly described would also change.
I didn't see "equilibrium zone" in there. It would help if that was defined since it might just be parts near the melting edge. Obviously if nothing but snow buildup is happening to the bulk of the ice sheet, then it doesn't much matter what happens in the "equilibrium zones".