It might also be 'simplified' for story purposes. They could speak a variant of it. In the same way that both Latins and Greeks have essentially the same name for their top god. The Latins have Jupiter, and the Greeks Zeus*, which is basically the same*. There could be a similar sort of connection, which Ayla inventing a name like Zeus, whilst the Zealots call it Jupiter - but the story retains Zeus, just to keep away from irrelivant linguistic considerations.
*Before anyone tells me 'No, they're just two different names for the same god!' let me explain. They are indeed the selfsame name. Instead of simply Zeus make it 'Zeus father' or, as it would be in Greek, 'Zeus Pater'. Mash it together. Zeuspater. Take off the ending 's' (which is an ending; the genetive of Zeus is in fact Dios, dative Dii) and you get Zeupater. Now, Z and I are a bit similar, at least in writing. Some Greek letters use an I shaped one for Z. Thus, Ieupater. Ie. Jupiter. Nifty, eh? Both languages have Indo-European roots, and the root for 'pater' is of extreme, extreme antiquity. Most western languages use a variant, from the English 'Father' to the German 'Vater', Greek 'Pater', and so on. And the Zeus-god is a very old Indo-European deity, part of both Latin and Greek roots.
