A change from a 9.6 to an 8.0 wouldn't be unheard of. Books can be damaged while being autographed. (A Signature-Series grade is a qualified grade, anyway; due to the writing, the universal grade on any autographed book would be much lower.) Some artists are careful when they handle the books that they're autographing, but some aren't. Books can come back with hand sanitizer spilled on them, coffee spilled on them, popped staples, etc.
What is weird is that they "found" the book so quickly. I guess the likelihood of that depends on how many copies of that issue were submitted to be autographed by that artist on that date.