I've noticed some performers are better at lip syncing than others. Lip syncing for television performances used to be more common than it is today, so many young performers aren't as experienced as older groups. Using a live vocal microphone over professional backing tracks is now a more common practice.
I remember groups used to lip sync over the studio track, which meant they needed to remember every extraneous shout or "ooh" or moan they performed on the original recording. Some performers would forget to emote on-camera as passionately as they sang on the hit song. Others would deliberately stay out of sync with the recording to point out the deception. I think the best performers sang along with the vocals on a dead microphone. Pantomiming the words doesn't seem to work as well.