Piast vs. Piastun

Am I missing here something? According to the myth, Piast and Piastun was the very same person and Ziemowit (Siemowit) was his son/
But in the movie, you have Ziemowit, son of Piast as an adult and Piastun as a totally separate person. Do you think this is only Hoffman's personal interpretation?

Iolaus: Have you ever been wrong?
Hercules: I thought so once, but I was wrong