These GW differences are not unusual. The W&B sheet is most of the time based on standard weights, e.g 75 KG per pax, or some fixed values for men/women/children. There is no surprise the actual values rarely match the canned numbers.
This should also make you think of what could be your real TOW when you take-off close to the limit...
Alpha call-up is a secondary tool not meant to correct the FMS values, but if by curiosity, I read 6.5 tons difference with the FMS weight, I would prefer to look again to the W&B sheet to search for gross errors or FMS input mistakes.
Another useful clue is to call up AOA. It has happened in the past that AOA sensors could be slightly miscalibrated, and the AOA1 versu AOA2 could show slightly different values (we're talking symmetric flight here, not engine failure!). If the two AOA sense precisely the same values, the W&B is mostly the reason for inaccuracy, or finger trouble while initializing the FMS. If there is a consistant slight discrepancy or more (like 0.3 to 0.5 degrees), I'd be more inclined to trust the W&B.
Funny to note that QAR (or FDM, whatever) seems to record the weights from FMS, so if you land at 64.5T from FMS, you could actually be higher. Unfortunately, this has given bright sparks the idea that they could modify the GW in flight when they see the predictions showing a LW slightly higher than max. Plain stupid.
Bear in mind that 1 tons difference gives about 2kt error at GD, and about 1kt error at Vref.
Flex