In Percy Jackson: Sea of Monsters from 2013 at the beginning it's clear that Percy Jackson is:
Son of Poseidon, god of the sea.
Also the whole plot suggests that.
So why does the centaur at 25min of the movie say that he is:
the only living half-blood heir of Zeus, Hades or Poseidon
The same at 1:35min:
only living human child... of Poseidon, Hades or Zeus
Why are they using the "or" statement? Aren't they sure which son he is?