I don't know, other than that they both had very singular events happen to them when they were young, and neither of which would necessarily factor into Sereitei's significance/insignificance as a force for protection.

And what was it that happened to both of them that was so singular?

For Rangiku, it was the syphoning off of her reiastu by Aizen. For Hisagi, he remained hidden and unscathed by Hollowfication.