I am guessing its because those personas are the highest rank of the fusion chart, so bringing out the highest of the corresponding Arcana. Levels don't really affect the Persona combination, but rather what they rank in the charts. Like say if you have a lv 99 Izanagi and a lv 99 Sandman, the combo will still make a Orobas regardless of level.
Hope that helps. --- AKA as Zoelius / PSN: Celis777 [ working on P4G Faq] P4A Main = Aegis / BB Main = Carl