[jaratec]

The way I see it, there are two factors for the origin myths:

The misidentification of various people. Practically every barbarian horde descending from euro-asian steppe into europe was called scythian as if it was one and the same. That was the case for huns, avars, magyars and others.

The desire to be associated with martial might/success. The desire to claim primacy (and therefore rights) over some territory. History is rife with such myths of origin.