That Venus Williams match referee error

Saw that match on a replay this morning since there was no play again all day today. It was quite bizarre actually. There was no point won, it was only a first service fault which was returned and the umpire just changed the score from 2-1 to 2-2.

What surprises me is that the players did not realize the score change…are they so focussed that they have no idea what the score is? Either of them??