Similar to how modern interpretations of ancient deities even change (mostly because of creative liberty in certain works of fiction where some interpretations of just stick with current trends like Heracles being seen as a hero when the actual Heracles was very wrathful), even ancient mythologies were just vast and interchangeable as cultures interacted with one another. Even as time progressed, the impact of these deities even changed like the god Neptune was first worshipped as the god of springs and rivers but eventually became worshipped as the god of sea and the ocean, or when ancient Norse and Slavic mythologies were later written from a different point of view during the Medieval Era like the Poetic Edda where the Christian influence is very visible (like the story of Baldur where he was loved by all of by the Norse gods except Loki, the recounts of Baldur's tale by Snorri Sturluson depicted him as a warlike-deity. Or how Helheim can be interpreted as the Norse equivalent of the Christian Hell due to its name and similarities but it was merely a place where the dead well. Or in Slavic mythology, the god of darkness, Chernobog, can be easily associated with evil, chaos, and villainy but this is due to the sources about Slavic mythology are mostly Christian interpretations which possibly influences how we view other mythologies because of the religion's own interpretation of good and evil and its own interpretation of other religions and their deities
Most of these stories were not exactly written on stone or written on texts that they were treated as law and different cultures had their own versions of the same gods with different names, origins and tales (like the god Quetzalcoatl has different names in many Meso-American cultures and to this day, nobody knows for certain about his origins and its relevance in most Meso-American cultures except that it was very prominent and impactful in those cultures. Or how different cultures blended together to even form different gods of their own despite the similarities in their pantheon (like the most powerful of the gods are usually associated with the heavens, the sun and the sky) like Hermanubis which was the combination of both Hermes and Anubis.
It is also possible that these myths and stories were shared by word of mouth so there is a chance that these tales have slight nuances every time that they were told to other people which probably made consistency of the tales and what the deities were associated with more difficult (such as Athena was regarded as the god of so many things – wisdom, strategy, craftsmanship, art and so on. There are many stories of Athena where she shared her wisdom and guidance to different people like when she gave Perseus a shield and told him how to defeat the Gorgon and yet, there is a story of where she lost her temper when she was defeated by a rude and egocentric human who was better than her at weaving and turned her into a spider as punishment)
So as we look into these mythologies and the stories revolve around these different patheons and their deities or other beings that are involved, how can historians know which stories were actually legit and are the ones that were mostly associated with the deities in question?
Source: reddit post