You may have put your finger on it there BOAC; rate of deceleration strongly affects stall speed, and piloting technique is a big player there. Were they the same TP?, Does the FTI (if any) show the same deceleration rates between about 1.3Vs and Vs?
I have seen trim make a big difference to stalling characteristics (particularly if there's asymmetry or internal freeplay in a trim tab design), less so to the actual stalling speed.
G