Riddle Me This...

So I have a question for all you wonderful women out there. I don't understand something and it has happened to me a lot more than I would like. I was always taught that you mean what you say and say what you mean so in that way we can communicate. Anyways this will make more sense after I say what happened and it is this... Why would a girl who is not single say yes to a date with another guy? I get the excuses that I seem like a nice guy, what else was she supposed to do... say no? I'm not angry but I would like to understand. If anyone could explain this to me, I would really appreciate it. I know they don't want to hurt me or lead me on but what do they expect. Heck, one girl actually went on the date with me and I didn't find out until later! I don't understand. Help me out please, because I have no answers and I'm not getting any. Is this my fault?