It annoys me along with a number of other examples of programme makers proving that they don't know their job.
Sometimes on "regular" programmes it seems to coincide with a change of director. Suddenly the camera swings from face to face just as each character speaks (or just too late, to specifically insult the lip readers in the audience). Alternatively, in hospital scenes, the camera follows the talking character, but the only noise you can hear is the trolley taking a patient, who isn't even in the story, down the corridor.
Another mistake they make when they try to "make it real" is, in a bar or disco scene, to have the characters muttering to each other, with music playing louder than they talk. This is obviously rubbish, as in REAL life, they would be raising their voice to be heard, and one of the characters would be straining to hear, and asking the mutterer to repeat themself as they can't hear a blooming thing.
It is all down to the director or editor being incompetent.
Quack!
GtD.