I just want to quickly point out that there's a difference between lip-synching (mouthing the words at the time of the filming), and post-production audio-sweetening and/or overdubbing.
The act of overdubbing the vocals on a line or two (or on entire songs) before the release of a concert video is pretty common. It's done to fix everything from a botched line, to a sour note, to a bit of feedback.
Concert videos are often regarded in much the same way as a film, where editing, overdubbing, etc., is used to make it the best possible product it can be.
Very few are ever completely "worts and all."
The other thing to keep in mind is that, generally more than one night of the show is filmed. Both Stevie & Fleetwood Mac's 'In Concert' videos from the early '80s were filmed over the course of two nights. As were the 'Tango' and 'Live In Boston' videos.
'The Dance' was filmed over the course of three nights... a dress rehearsal and two audience-attended concerts.
So audio from one night and video from another are often intermingled throughout the released product... which sometimes causes a discrepancy between what you see being sung or played on stage and what you hear on the soundtrack.
It's not a case of lip-synching on stage, but rather post-production overdubbing.
