From What I can figure. the Primals are "Embodiments" of the element By their Worshiper, so it only goes to assume that figure would take on the form of what that culture believes it to be. While channeling the energies of fire. one tribe may imagine Ifrit. while another across the sea may picture him as a Giant lizard. and a third still may picture a giant baby who farts flame. it is all in their eyes. so when the energy manifests that is the form they give him. thus explaining the overlapping elements.