Is there somthing I'm missing?
I've not flown the 763 so I am unsure of the theoretical maximum fuel load - however I would imagine that VORDME2 is saying that because of the high temperature of fuel in BKK the volumetric limit of the tanks will only allow 69.9 tonnes. In Europe where it is significantly colder the volumetric limit of the tanks will allow 72.5 tonnes.
Regards
Exeng