5 gallons of liquid does indeed occupy a volume of 0.67 cubic feet (5 gallons divided by 7.48 gallons per cubic foot), but a "5-gallon" bucket actually holds a bit more than 5 gallons. Take your bucket and a 1-gallon jug, fill the bucket with water a gallon at a time, and mark the side of the bucket at each 1 gallon interval. You will discover that the 5 gallon mark ends up about 1.75 inches from the top of the bucket. Therefore a full bucket really holds somewhere in the neighborhood of 5.75 gallons (+/- a few ounces, depending on bucket manufacturer). This "extra" space in the bucket is there for practical reasons, as well as for safety--imagine the mess potential in opening a bucket of paint that is full to the very tip-top. So if you are filling your bucket to the top, you are really dealing with a volume of about 0.77 cubic feet.
Like i said one of those is not 5 gallons :D 5 Gallon buckets hold more then 5 gallons but we just call them 5 gallon buckets instead of 6 gallon buckets or whatever.. :D