User blog comment:Aizen.sama/Are Espadas, Vasto Lorde(s)?/@comment-1393830-20100420213220

Tosen gained a perfected hollow form from Aizen using the hogyoku. It has been said that Aizen knows how to use it better than Urahara did. Ichigo's form is natural. These two are very different from the other vizard and thus it could be that their different circumstances of creation allowed them easier access or access in general to a ressurection