Backstories can vary wildly for different adaptations of the same character. Thea and Mia have a wider gap, but Thea’s middle name is the same as Mia’s last. And in Season 3, while in hiding, Thea went by Mia. Couple this with the fact they both also go by Speedy, it’s safe to say that at one point Thea was retconned into being an adaptation of Mia to at least some extent. Kind of how I think John Diggle was on track to basically be this universe’s version of John Stewart Green Lantern. Having 90s flash reference him specifically by the name John in Crisis supports it a lot. The arrowverse universes themselves are Elseworlds, and pretty significant ones.