Board Thread:Questions and Answers/@comment-10797017-20130627100621/@comment-11051098-20130820181534

Next level bankai wrote: I use to think the 5 SWP were one of each category: Human (Ginjo who just appeared with the Shiba clan in Soul Society) Shinigami (Kenny) Hollow (Grimmjow to be the real reason SternRitter over took Hueco Mundo) Quincy (Uryu) and Ichigo who is all of them but then I forgot Aizen was one of the five and threw my theory out in the water

If this theory would have been true, I would think they rather took Hueco Mundo because of Harribel. She was confirmed to be a Vasto Lorde, and she was ranked 3 places above Grimmjow as an Espada. I would think she has far greater power than him, unless the Espada were ranked after their level of reiryoku and not actual power.