It needs to take into account that some fish, even if they're small, don't do well in small tanks. In MyAquariumGuide , I put a molly in a 10 gallon tank and it said it should take up 2 gallons. So at least one molly + other small fish + shrimp perhaps should do nicely in a 10g? Mmm, no. In my experience, that works horribly. I had a molly, 2 platies, and a shrimp in a 10 gallon and after the initial excitement of new fish wore off, I realized that I knew deep down they weren't happy in this setup. I returned the molly to the store and got two more platies to add in her place. The four now seem to be happy as clams, and healthy too. The whole tank used to be riddled with health problems while I had the molly in there. Imagine if I'd initially added mollies to my tank according to MyAquariumGuide 's suggested ratio of 1 fish:2 gallons...
MyAquariumGuide really needs to look at the specific needs of each fish more in depth. Some produce more waste than others. I think MyAquariumGuide would do good to change the minimum tank size for one fish of a species to the min. tank size for the least # of fish in a school required for them to be happy. (Mollies really shouldn't be kept on their own without others of their kind, and given they're livebearers... Yikes! You're definitely gonna need more than 10 gallons.)