[SPOILERS] Why is Ragnar's last name Lothbrok ?

As far as I know its was a tradition of the northmen that sons get their last name based on their fathers name. In Ragnar's case he should be named Ragnar Sigurdsson (because his dads name was Sigurd). Also why are they calling Ivar, Ubbe Sigurd, Hvitserk Lothbroks but only Bjorn Ironside ? Their names should be Ivar Ragnarsson, Hvitserk Ragnarsson etc.

This might be nitpicking but it still kinda bugs me...