I have nothing to add here but im going to throw out a guess so if I'm right in a few months I can gloat or something.

I'm guessing it is possibly the error correction stuff on the RAM allowing cards to seem functional at high memory clocks meanwhile the ram is way past "stable". Perhaps they can even get past internal QA like that.
I remember reading some review that said the ram would correct errors when unstable and overclockers would have to look out for lowered scores. That leaves me thinking though that a small to moderate amount of instability would be nearly indetectable without a direct measure of the memory bandwidth.
Remember how they also said the ram auto negotiates some of the stuff with the controller on startup? maybe the ram keeps running at the higher clocks while correcting the errors, but on reboot it cant negotiate the settings with the controller?