Confused with last name Doskje?!?

My great grandfathers name at birth was Olav Aslakson Doskje. This was on his birth and conscription papers. It also said he was born on Sollid u.Hauge in Kviteseid. So where does the Doskje fit in. I have read the data on naming practices and am still confused. If they are supposed to take the name of the farm.....why is it Doskje and not Sollid??