They don't only sound similar, you know. It clearly did its job considering how many overlooked it.
It's not all that strange to have several gods that are roughly the same with different names. For example you can look back at the Roman Empire, they incorporated all foreign gods with their own religion and merged them as far as they were similair in principle.
I'm a little short on actual examples right now.
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Brian, it's not a bad idea, but a chart will be over looking the edges of each individual god. The Cyrodiilic Kynareth for example can be seen as an combination of the Nordic Kyne and Altmeri Y'ffre because Kynareth is both the goddess of the sky and the woodland creatures. (I'm aware that the unification isn't described anywhere but the spheres of Kyne and Kynareth don't exactly overlap).
A foot note would be helpfull.