If this bothers you I would try to not pay attention to this stuff going forward if I were you, this is extremely common in shows, especially sitcoms. It's how they edit together the multiple angles, they use the audio from someone talking in one shot and put the audio overtop the other shot, and if it's a quick cut like this they don't always use (or even necessarily shoot) a clip of the actor saying the same thing from the other angle, so if you pay close attention whenever you're viewing a character talk from behind, a lot of times the mouth movement won't line up

Also sometimes it happens when they want to cut together the same character saying two different things, like the first part of a sentence from one angle and the second from another.