The grader notes don't mention much in these higher grades, so that's expected.
I have seen that damage, it's a dent if the corner hits anything. That could have happened if the person from your comic shop dropped the book at anytime, or by accident hit the corner of the book against the box he was shipping it in (along with other submissions). It could have happened during travelling if the book was not secured properly and hit against the side of the box. It could have happened at CGC, though that one I doubt because usually they tell you if something like that happens.
Best guess is it happened between you handing it over to the comic shop and them sending it to CGC. Did you opt to have your book pressed by CGC? Because that would eliminate it being the comic shop's fault. If not, then it's probably the handling of the book when preparing for shipping (or during shipping). It didn't happen once enclosed, pretty sure of that.
CGC then graded it accordingly, giving it a 9.2 due to that corner dent.
And cases are easy to scuff, so that could have happened anytime. If your comic shop recieved lots of submissions back, then cases rubbing against eachother as they were put away and stacked together could have caused that.
The good news though is that this can be fixed with pressing the book, so you could still get your higher grade. But you'll have to spend to get it pressed and regraded (which would now be an extra $60)