Why are the PureOS names different?

found this on a web page and it didnt have an answer, and neither do i.
I still do not know what is the scheme behind naming of the download ISOS “Prometheus” and “Hephaestus” and why they are not using “Amber” or “Byzantium” names instead.

It was asked before then there was this reply though I don’t think it fully answers the question how they relate:

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thanks for that but as you say it doesn’t answer the question