It could be said that fundamentalists of both religions subscribe to the most literal interpretation of their scriptures. They aren't the only ones 'actually following' it, they're just the only ones following it in the most literal, fundamental way. Other people follow in other ways. There's pluralism in religion - multiple ways to follow the same faith.

Problem is that within some religions there are sects that deny other sects the right to practice their interpretation - Exclusivist sects. Some sects are very aggressive in their denial. Sects like ISIS.

Nobody can say that ISIS aren't really Islamic. They've got their own interpretation of the scripture. It may be a shit and destructive reading of the scripture, but it's still a reading.