There are a combination of factors. Tangs eat a LOT and poop a LOT so small volumes of water are undesirable and even larger bodies of water should have a skimmer that OVER skims. Also, tangs are very active and need lateral swimming space (gallonage is a poor estimate of a tanks "tangability") because their natural range is so large (perhaps because of competition for food?). In any case the figures given are estimations and tend to be LOWER than optimal tank size.