I don't think such guidelines should be taken seriously. It's not like your fish is going to die if it's in a tank that's 5 gallons less than the recommendation... The guidelines are simply just that--recommendations.
However, if you really want a fish to reach it's full potential/size, you can't put them in a small tank and i think that's what these recommendations revolve around... You aren't going to be able to get an angelfish to grow 6" in diameter if you keep them in a 15G. Having owned angelfish myself, i personally wouldn't put a pair in anything less than 40-55G.
Who decides these guidelines? Who knows. Maybe chain pet stores, or maybe some of them are just based on a general consensus...
Just my opinion.