User blog comment:Matarrok/Too many gas or airbased devil fruits?/@comment-2031609-20111023070518

Well the problem kinda lies in the fact that we haven't made a guideline for how close they can be, or how general or specific they can be, so as long as there is a slight difference people can get away with it. Like how we have a helium, a gas and an air logia, which are the same basic concept, but are completely different because they are different material. The same goes for the fire/heat based DFs. People can get away with different Fire logias because they are not strictly fire, but a specific type of fire (hell-fire, cold fire, fiery inferno) so they are considered different, though they are all fire. If anybody is interested, who doesn't already know or forgot about it, there is a DF forum to discuss DF issues. He's the link: http://shipoffools.wikia.com/wiki/Forum:Devil_Fruits.