Board Thread:Questions and Answers/@comment-10797017-20130627100621/@comment-11051098-20130628162618

I think Yammy was ranked strongest in his release form not because of overall power - I doubt his reiatsu surpassed the top 4 - but because of the immense physical strength he seemed to have. When he was attacked by Ichigo with his new mask, he only got a minor cut on his neck, even though Ichigo only had a fraction of his full power back then.

As for the reason of the Vandenreich's attack on Hueco Mundo, Harribel might have been strong enough to pose a threat. Almost 1,5 years passed since Aizen cut her down in the FKT, she might have trained and done what she once refused to - devour other strong Arrancars and Hollows and increased her own power.