Eiruvin 13b recounts an argument between Beit Hillel and Beit Shammai. The argument grew very heated until a bat kol intervened and stated the famous line elu v'elu divrei elohim chayim ("These and also those are the words of the living God"). But then the bat kol concluded v'hahalacha k'veit Hillel ("And the halacha is according to beit Hillel").
The bat kol seems to settle the dispute; however, we learned in the story of the Oven of Akhnai (Bava Metzia 59b) that we don't listen to a bat kol to settle halachic disputes because the Torah is lo bashamayim hi ("The Torah is not in heaven").
So why was the bat kol able to settle this dispute?