User blog comment:MasterBroccoli/Civilization Categories/@comment-5157002-20161225081631

Originally, only the AoEIII and AoM civs had categories - it was decided that there weren't enough unique aspects to the AoE and AoEII civs to give them their own categories (usually, just a single unit and two techs - not even that in AoE). That's why the categories note that they're for AoEIII civs.