Glee 3D concerto Movie - Valerie.

I don't understand why Valerie is so obviously lip-synched in the sneak peek that was released the other day. I went to see the concerto in Dublin and it was definitely live, as it seemed to be on the anteprima we got of the movie soundtrack. But why would they replace Naya's live vocals with the studio version? Did they just paste the studio version over the sneak peek so that people won't be able to download the live version until the song is officially released, but will keep the live vocals for the actual film? Does anybody have any idea? Thanks in advance for any input.