Most dental fluorosis cases are mild, so the mild case is more typical. Cases such as the one shown in the severe image are quite rare. There's nothing questionable about the mild case; many cases are milder than that. The Colorado brown stain came from water naturally fluoridated to levels well above recommended levels, and it was more severe than what is typically experienced nowadays. Today, most dental fluorosis (in western countries anyway) comes from too much toothpaste.