OMFG !! they actually calculate based on the outside....
This is common in all areas of marketing, I used to sell houses... A 1 story house that is measures on the outside 26 x 50, now that's a 1300 sq foot house, lets not worry about the 6 inch thick walls on each side (thats 1225 now), and the 5 inch thick walls on the inside (thats 1152 now).. Closets, cabinets, pantry, laundry!! thats 950 sq feet of living space now...
Did I mention I used to sell lumber?
And that container of ice cream is not 1/2 gallon, check the label.
I guess tanks are sized by the amount of water they would displace, not hold..
Seriously though, that's a great reminder for dosing.. I wouldn't worry too much on the filtration or heater, as over sized is not generally a bad thing..
Y'all keep in mind all the "other stuff" in the tank to determine your actual water volume before dosing too...