Hi birdistheword,
Welcome to the Heartland Owners Forum. I hope you're enjoying your BC.
Obviously the 20 gal notation is a marketing statement intended to convey the information that this water heater produces the same amount of hot water as a normal 20 gallon tank because of the faster recovery. Alternatively, you could look for a tank that holds an actual 20 gallons of water, but has a slower recovery, also providing 20 gallons per hour of hot water. The alternative wastes space, wastes energy keeping twice as much water hot all the time, and adds weight and towing expense.
Most people just want to take a hot shower. And if you get twice as much hot water, do you really want the bigger, heavier, wasteful solution?