What were these girls thinking?

I'll make this short. I asked girl A if she wanted to go to a dance with me as a friend, and at first she said yes. But the next day, she said that she wouldn't be able to. A while later, I asked girl B if she wanted to go to the dance, and she said yes. When I arrived at the dance, girl A was there, and girl B completely ignored me. After the dance, both of them acted like nothing happened.

I was pissed off for a while, but I've forgiven them. What I don't understand is why they acted like this. Was there something that I did wrong? Was this behavior justified?