> Many of the exceptions will be obviously foreign names with traditional spelling, which can go either way

Or the names of biblical prophets like Ezra or Elisha. Not very popular today but you can find a lot of them in 19th century records.

Those names were longer in their original form, so none of them ended in "-a", but in English one or more letters were dropped, resulting in the "-a" termination.

E.g. Ezra was Esdras in Greek and in Hebrew it probably ended in "-ahu", while Elisha was Eliseus in Greek and in Hebrew it ended in ayin after the "-a-".