Let's also not forget that no one is ever at 100% all of the time and everyone can have an off night - maybe Chris talked her up thinking she was going to come out and nail it, and then she simply had an off night. The voice is an interesting instrument - I've had nights where I have just nailed it, and I have also had nights where I was all over the place and couldn't really put my finger on why. (edit: "all over the place" for me vocally is still passable to anyone who really doesn't know what to listen for.
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Let's also not forget that bad sound and monitoring can destroy even the best of vocalists - if you have spent any time doing lead vocals on a mic, you'll know what I'm talking about. When you get up there to sing and you can't hear for diddly, the best you can do is lay it out there and hope that the practice you put in is enough to keep you near enough to the pitch so that you aren't completely awful.
And lets also not forget that there are times when who you are or who you know goes as far or farther toward getting you the gig as does your talent and abilities. I know I'm probably going to get blasted for saying this, but Sunday night I was at a Van Morrison concert and I got to thinking that if he wasn't Van Morrison - who sells out venues whenever he decides to tour - he might have a hard time finding a gig. His sax playing was...well, sharp and thin, and while he certainly has his own vocal stylings, it isn't really anything that my cover band could use, unless of course he was going to be hired to cover Van Morrison type tunes. And he wasn't really engaging as an entertainer - he never even talked to the crowd except for one time when he announced a guest vocalist. (the dude who opened the show for him) He just sort of stood up there in one place and sang his songs. Then again, he might have just been having an off night!
Keeping in mind I didn't see the program, I'd be willing to bet that when this girl is on, she is a fantastic singer, and that due to her age and lack of experience and polish, she probably just had an off night.