I heard someone say in an interview on the BBC (or maybe CBC or C-SPAN) that there's non-Koranic stories (Hadith (?)) which various Islamic sects give various levels of credence. Some are argued to be false by some groups. That's why there are different interpretations of "jihad" and why some can argue that external, armed jihad is a responsibility of Muslims while others can argue that killing one person is the same as killing all humanity.
See, e.g., [link|http://www.muslim.org/books/f-ahm-mv/ch10.htm|this] discussion.
It's not just the Koran, as I undersand it. But I'm no expert...
Cheers,
Scott.