See Joan Turville-Petre, "The Genealogist and History: Ari to Snorri", Saga-Book 20 (1978-81), pp. 7-23 (pdf), especially p. 15: "[Harald Fairhair's] successors Óláfr Tryggvason, Óláfr helgi and the Norwegian kings descended from Haraldr harðráði [e]ach . . . represented a separate line, which had to be connected to Haraldr: as he was their political forerunner, so also he must be their common ancestor".
