That is worrisome. You should contact MoreBeer to let them know. They shouldn't be labeled as 5 gallons if they can't hold 5 gallons.
No kidding. I have to assume that the 2 1/2 gallon ones don't hold 2 1/2 gallons which kind of kills it for me: I am starting to lean in the direction of going a bit bigger (3 gallons) not to a bit smaller.