minibrew volume off?

Been using my min-brew for a couple years and always took the volume for granted. Until today I was getting ready to make my first wine batch and decided to double check the markings. Man, mine are a half gallon off after the first gallon. It seems like 6.5 gallons would fit only if I filled it up to the very top. Anybody else have this problem? I put it on a level area and even broke out the level to make sure I had the thing positioned well and then double checked my gallon jug with 4 quarts. I've been going by the giant 5 for my beer batches. No wonder why I've had the efficiency of a rock star-- less topping off water! Anybody else notice this?