Volume equivalents are supposed to be based on FF black powder - i.e. a measure set at 100 grains should throw a load of FF black powder that actually weighs 100 grains.
I've tested a number of powder measures and the TC measures always throw a lightest load. The TC U-View measure is the worst, typically throwing at load that is 10 grains light - i.e. when set at 100 grains it throws a load of GOEX FFg that actually weighs 89 grains and a load of GOEX FFFg that actually weighhs 90 grains.