In-universe it was a summoning gone horribly wrong that ended up in both goddesses getting summoned with the same body (and part of Rin's personality melded into theirs) and part of the plot twist that Ishtar was actually not one of the Three Goddesses Alliance that serve as "antagonists" (quotes are here for a reason, aside from maybe the Gorgon none of them are any kind of bad) for the first part of the chapter.
But really they probably just wanted more Rin. It's not really something I can disagree with.
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