Whenever girls say something like this, especially when you haven't even asked them explicitly about getting into relationship :
"I want (just) to be friends"
"You are nice but not my type of a guy, we can be friends if you want"
"I would like to have a boyfriend like you but it's better that we become friends"
they do not want you to be friend with them, because all of these things sound like "you just aren't a worthy person" and they very well DO know how that sounds. It simply isn't a way to start a genuine friendship. And while intersexual friendships do exist (although those are very rare), I guarantee you not a single one started like that. Friendship is something that includes respect and starts spontaneously, not with explaining your "friend" how unworthy he is.
What these girls want is an ego boost, they want you to constantly ask them about getting more than a friendship and of course they'll constantly reject you and brag around to their friends how they have guys who would like to be with them but they are just too worthy.
And if you start treating her like your other friends which means joking with her, asking her for help or similar she'll suddenly lose interest in "friendship" and run away without even explaining why.
Conclusion : if you stumble upon a girl who is an egomaniac just start treating her as a real friend and she'll automatically leave, you don't have to ask yourself if you have done something wrong, because it's up to her not up to you. Such person will never be happy in her life and you better not become part of it so consider yourself saved.