We have e.g. "Brother Jim" or "Sister Jane", which implies that they are equal to a person of any status, or "Father John", which even implies a certain superiority. And even the translation of Asha'man with Guardians, implies that an Asha'man is more powerful than you.

Thus, it's a bit curious that the proud and aloof nowadays Aes Sedai still demand to be called servant by lesser persons, although the title "servant" automatically implies a lesser status.

But of course you are right that it's in tradition with the AoL, and therefore understandable (though other traditions were changed as well).