If Queen Gunnhildr IS the example we should follow, the title was given to the best astrologian of a convent of women specially trained in that, correct? The fact that she was a Roegadyn in a city overwhelmingly populated by Hrothgar was an anomaly, as the queen had always been a Hrothgar to that point just by probability.

So the Bozjan based clan, at least back then, chose a queen based entirely on who was best at a relied upon skill.