They can call it a 12 as in it takes up that much space including lights.
They can call it whatever they want.
They always round 27 gallons up to 30 and also when you buy a large tank, they take outer dimensions and that's what they call the tank. For example a 6 foot long tank, 24 inches high and 18 deep they will say is 135.
But the half inch of GLASS to form the walls and the overflow that sits 2 inches from the top of the tank means the tank cannot possibly hold more than 115 gallons.
Yet they can call it a 135. They can call it whatever they want.
Yours was prolly ALL equipment included. The total dimensions, including the light fixture, calculate to be the equivelent of 12 gallons.
It's a scam, but it's legal.
If I were you I'd call them and complain, tell them you want a tank that HOLDS at least 12 gallons, and you feel as if you were mislead. Maybe they'll replace it with the 24. Can't hurt to try.