That was live you could hear her breathing between notes.
this is why it's called pre-recording everything must sound live she has done many performances like that (all her previous AMAs performances) and I don't even mind its better that full play back like she used to do back in the day or bad vocals and many don't notice.
But mess at you guys thinking that was live. It was obviously prerecorded kind of like Cheryl Cole's performances. It's still her voice though but just not live and obviously enhanced.