http://www.psacard.com/pop/baseball-...52/topps/49722
Cream backs or unknown*: 894 graded by PSA, 2 PSA 9s highest on pop report
Gray backs: 18 graded by PSA, 2 PSA 6s highest on pop report
So once this gets reholdered, it will be the highest graded gray back as mentioned above. That would account for the additional sales price. Gotta figure the consignor may not have known either, or he would have resubmitted to PSA beforehand.
*: This variation was added after they had graded at least a few hundred, and there could still be gray backs hidden among them.