I've noticed that in almost every TV show or movie I've ever seen, the audio does not sync up with the actor's jaw/mouth movements when the actor is being filmed from behind. It looks as if the actor is saying something completely different than the audio I hear. It's shocking how universal it is - pick any live-action content and you'll see it at least 90% of the time. Reality shows, comedies, action, it doesn't seem to matter, they're all affected. It's worst in quick cuts. Why is this?
Because the sequences are edited. You may be watching an edit assembled from several takes and entire lines of dialogue may be omitted at the editing stage.
On some shoots, there may only be one camera. This means the scene has to be shot from one angle, performed multiple times, then the reverse shot set up, performed multiple times and an edit assembled, usually from the preferred performances, but sometimes combining different takes if there are any problems.
The use of cutaways and “noddies” - nodding interviewers - is a telltale sign of editing in interviews that most people don’t notice, but once you’re aware of how video is put together, you notice it all the time.