facts....It's actually 100%, because attraction is not a choice. The failure in understanding here is assuming that because she didn't do it with you/me, then she never does it. There is always a guy out there who is attractive enough and appears at the right time and she will do it. If she hasn't done it yet, then the potential always exists for that right guy to come into her life.
If a girl I date doesn't bang on the first date, then I don't default to "Oh she's just a good girl who doesn't do that on the first date." I just assume that she's done it/will do it with another guy who turns her on more than me.