SG of Jet A-1 v's Jet A
As others have said the primary difference in the two types of fuel is the Freezing Point, Jet A freezing at a higher temperature than Jet A-1 and Jet A-1 has an anti-static additive. There is no measurable difference in the SG of types if they are both at the same temperature. Each has a nominal SG ranging from 0.78 to 0.82 and if you uploaded an amount of fuel which should have resulted in a predetermined weight of fuel on board, after refuel, but had a discrepancy you need to look at why. Did you check SG of fuel in aircraft tanks before beginning to refuel? Did you check SG of uplifted fuel? Are you sure you knew the amount of fuel in tanks before uplift? There are many variables to be considered.