15, 16, 18-20, is one of the many later scribal additions to the book.
"Introduction to the Old Testament"
John Edgar McFadyen
Haec restores sense to the passage, and was the preferred reading of Heinsius; I consider it a scribal conjecture, now rendered obsolete by Professor R. J. Tarrant's more elegant quae.
"The Last Poems of Ovid"
Ovid
It is possible that the occurrence of Guthlaf as Garulf's father is simply a scribal error.
"Beowulf An Introduction to the Study of the Poem with a Discussion of the Stories of Offa and Finn"
R. W. Chambers