Why were the three sons of Anak still alive if the city of Hebron had already been conquered previously?

According to Joshua 10:36–37, Hebron was attacked by Joshua and the armies of Israel who destroyed all the inhabitants together with their king. One can assume that since the campaign took five to seven years, the city was re-inhabited at some point after the battle. It seems as if the three Anakite leaders, who are also mentioned in Numbers 13:22, somehow survived the initial battle for Hebron, only to later be driven out or killed by Caleb.¹