Mmmm, I've ran into similar things this gets into something very diversified and because its gets so wide spread things its like finding all the connections in a cotton ball. The choices are
independent it just came about similar
same the same story bought name changed with different group
Foreign Influence parts of one story added to another.
Reality vs Fiction reality similar to or mirrors fiction or fiction based on event.
prehistoric ancient story archetype that is constantly re-imagined.
Some kind of biological need. We feel compelled to tell similar stories for reasons unkown.
An example of this kind of mix is Santa & Mrs Claus & Black Pete. It Starts off innocent enough with stories about the person Saint Nicholas, and a poem about Goody Santa Claus. And of course we've all added to that with Rankin & Bass cartoons and Tim Allen movies but if you back track the stories they are much older possibly neolithic or older dating back to bear cults. Furthermore, there is strong element of using them as superstition based social controls.
Basically its a role being fulfilled not the actual character.
Santa Claus: Saint Nicholas, Kris Kringle, Father Christmas, Odin, Thor, The Huntsman, and even Black Pete (now a companion)
Mrs. Claus: Goody Santa Claus, La Befana, Perchta, Berha, Holda, Bridget, Freya, Snow Maiden and a male knight named Artaman
Black Pete: Zwarte Piet, Satan (avoiding all his names), Knecht Ruprecht, Krampus, Belsnickel
Santa's Reindeer: Goats, Sleipner, and The Wildhunt
Elves: Gnomes, Dwarves, Krampus, Perchten
And then there's the whole Noah & Gilgamesh thing as well as Balder & Glooskap. We are all one people and our stories are told retold lost rediscovered and mixed.