Did you look to see if the number is the same as your GIA report? My guess is that maybe it is the appraisers number. But that GIA number certainly should be on the appraisal somewhere.
I agree with Heidi too. I imagine that it's a method used by the appraiser so that they can locate your appraisal in their system in the event that it becomes lost or you have questions about it later? Just a guess though.