lip synch - dialogue - audio transparent screens

What is the source of the time differential between video dialogue (actor's lip movement) on the screen and the audio signal (spoken words) from the speaker system? Is this due to signal processing?

I understand that hiding the center speaker behind an audio "transparent" screen aids immersion in the A/V experience by removing the visual cue that you are listening to a speaker. Centering it in the middle of the screen further makes good sense, as does placing it a foot behind the screen to avoid movement of the screen (seems to be a manufacturer "must").

What about listener - speaker distance at 10 - 15 feet from center? If the mains are a couple of feet further back, would that muddy the dialogue? What electronics "fixes" are recommended for dialogue differential?