I don't know, but I've been collecting slabbed GA books since the middle of 2000, and I have never seen slabbed books that were as over-graded as the books in the Promise Collection. And many collectors, including board members, were paying record prices for 9.6's that should have been 8.0's, 7.0's that should have been 5.5's, etc. I've never seen anything like it. It's like seasoned collectors lost all sense.
I collected raw GA books for many years before I got into slabbed books. The grades on the Promise Collection books wouldn't even have been tolerated back in the 80s when dealers' grades were all over the place.