4.A.1. The entire score must be called before the ball is served. If the server hits the ball to make the serve while the score is being called, the referee or opponent shall stop play, announce a replay, and then recall the score to re-start the 10 second count, allowing the player to re-serve without penalty.

4.M.   Service Faults.  During the service, it is a fault against the server resulting in a loss of serve if : 

4.M.11  The server hits the ball to make the serve while the score is being called.

4.A.1. The entire score must be called before the ball is served.

4.M.   Service Faults.  During the service, it is a fault against the server resulting in a loss of serve if : 

4.M.11  The server hits the ball to make the serve while the score is being called.

The reasoning behind the current rule is, I believe, so that the server cannot win an easy point by quick-serving their opponent when the opponent isn't ready.  Now that the referee is looking at the receiver while calling the first number of the score, and determines that the receiver is ready, the server is no longer able to get away with a quick serve.  Now when the server serves while the referee is calling the score, it is due to the server being too eager, not from trying to win a cheap point.

OR, sometimes at a noisy venue, the server thinks they have heard the referee call the score, their timing gets off and they serve while the score is being called.

OR, in addition, entry level tournament players often serve while the referee is calling the score, usually because they haven't worked with a referee very much.

Noisy venues, indoors and outdoors

Anytime the players are eager to serve

New tournament players

Wheelchair events - where the players play in tournaments infrequently and are eager to get the rally started.