Too true! as the Antipodeans say. A lot is interpretation. The Cantigas, as done by Jordi Savall's group, and others I have yet to investigate, have a lot of scholarship behind them. They are presented well on some of the recordings, but I don't think any of them would label their efforts as the one true interpretation.
I really don't know ancient music. Just fell in love with the sound of the old polyphony and have extended to a tiny bit of the earlier material.