Tanks dont hold what they advertise?

This post is out of curiosity - I'm curious as to how widespread this is, and what others think. I bought an 18 gallon tank. I got it home, started filling, and after 15 gallons it was full. I called the manufacturer (Top Fin), and they said that they did not even make an 18 gallon tank. They told me how to determine gallonage - multiply heightXwidthXlength in inches, and divide by 231. I did that, and came up with 15 gallons which is how much water it holds. Years ago I had a 15 gallon tank, and it was about the same size as this.

So, last weekend I found a 50 gallon tank at a garage sale. I get it home, measure the inside dimensions, and come up with 44 gallons. I measure the outside dimensions, from the very top of the plastic frame to the base, length and width of outside of plastic frame, and came up with about 50 gallons.

Is this a common practice? To advertize tanks as X gallons, when in fact you can't actually put that much water in them?

In other words, the advertised gallons should be viewed not as capacity, but rather as displacement.

As I understand it, one of the things that affects the physical size of tanks is the shipping - a manufacturer that makes and ships a variety of sizes wants to be able to nest tanks inside one another to reduce shipping costs and space requirements.

That's an interesting question. Maybe when they calculate capacity they measure based on the dimensions of the top and bottom bands and assume the tank is filled to overflowing. If you buy a piece of 2 by 4 lumber, you actually get a board that's about 1.5 by 3.5 inches.