"it is juat an approximation. A gallon of water weighs about 8-1/4 pounds.one cubic foot of water weighs 62.4 pounds. a 20 gallon long contains 2-1/2 cubic feet,or 18.7 gallons, when completely filled" - The Guide to: Statring Your First Aquarium
maybe the nanos are different..... but if you read the above it make a lot of sense....