Nerves can sometimes play a part. I find sometimes when I sing a song out for the first time, I sing it much too fast and it loses something in the process. I think it's sometimes a matter of get through it before you forget a bit or make a mistake. So the next time, I have to make a conscious effort to slow it down a bit so that I have time to sing the song properly.

The same happens with session tunes as well, sometimes.