Because of the Crisis on Infinite Earths, the five remaining universes merged, but there were anomalies like the existence of Huntress and Powergirl. It tried to refit these heroes that should not exist into the new history, and suceeded normally, making Huntress a Bertenelli, for example.

But it didn't work with everyone, and Power Girl's origins kept changing as reality tried to work her in somewhere. It's possible that these reworkings of reality caused Jason Todd to return to life somehow.