It could be you think angels is like it was in your dream but really totally different in real world. I do this every time I join in with a song on the radio, convinced I know the words but when my mouth opens it's nothing like it
That happened to a friend of mine, slightly differently though...
He drunkingly passed out on a chair and the song "Your Beautiful" by James Blunt came on the cd player, he started singing it in his sleep and he doesn't know any of the words to it till this day, he thought we were winding him up, pretending that he did it when he thinks he couldn't have, until he saw the video. I thought he was going to pass out!