Patrick is right. Difference in volume measures and other variables (including moisture content of the powder) will affect the result but 100 grains volume of T7 FFg will weigh close enough to 80 grains to make that a useful number.
Perhaps only rumor, but some say the value of 77.7 grains is the basis for the product's name.
T7 FFFg will weigh more per volume due to the finer granule size.