So, 3% variance. Assuming 224oz is the correct amount, you were shorted a few bucks worth of ink, but not 1/4 of the gallon. That assumes of course that the 224oz isn't an overage, in which case over time your variance probably gets close to averaging out. I wouldn't expect much variance at all though honestly...
edit: Find a quart container (one you know is exactly a quart), and weigh the container, then fill it completely with ink (and make sure you shake it etc to get out any air) and weigh it. Multiply to find out how much a gallon should weigh, then empty one of the gallon containers completely (use ink degradent etc) and weigh it. Now you should know how much it is supposed to weigh in total, and how much you were actually shorted and/or the overage on the other gallon. Would take a little time, but at least you would be able to figure out if they have an issue with how they are packaging their ink.