How hard is it for a girl to say no when asked out by a guy she's not interested in?

Over the course of several days I asked a girl out.

On the first day, I suggested an activity that might be fun for the both of us to do. She agreed it would be fun.

On the next day, I checked her availability (she wasn't available the day I suggested), then asked for her phone number. (We'll say this day was Thursday).

Two days later (Saturday), I called and suggested we go out to Dave and Busters on Thursday the next week. She agreed, and I told her I would contact her later, or we could catch up at work to figure out the details.

Tuesday night I know she's in class, but I call and leave a message. She never returns my call.

Wednesday night I know she's working, so I text her asking if 6 o'clock was a good time to meet for our date. She never returns my text.

Today (Thursday, the day of our date), she texts saying "Hey, I don't think it is a good idea for us to hang out. My boyfriend wouldn't really like that. I would have said something at [work, when you asked] but you kind of caught me off guard. I hope this doesn't make things awkward."

I'm 26 years old, and have asked out a lot of girls and been turned down plenty of times. I'm unaffected by it now, but it got me thinking. She has said within the past 2-4 weeks that she doesn't have a boyfriend, and I tend to believe she still doesn't. I think the "boyfriend" is her way of trying to reject me without hurting my feelings. But as you can see above, I asked her out slowly over the course of several days and gave her plenty of opportunity to say no (or to inform me of her boyfriend, if she actually has one).

So back to my original question: How hard is it for a girl to say no to a guy she's just not interested in? Is there another reason this "boyfriend" manifested itself days later?