It's all zuckerberg's one meta-house remortgaged a million times...
Is AAA even meaningful? Shit ratings were a major part of the 2008 meltdown, but we've all collectively decided to ignore that and keep relying on the big three's Divinely Inspired Appraisal. Sounds odd.
It is only meaningful in a relative sense - AAA is less likely to be allowed to fail (or maybe actually fail?) than BBb or whatever. Usually.