[just another ref]

I agree that the call is made at the spot. However, I see nothing that undeniably binds the signal and the call. In any other case, we can say "Oops, I made the wrong signal. This is what I meant." Yet in this case be are required to stick with two calls when it is impossible for the two to happen at the same time.

Consider the following:

A1 drives on B1. B1 has good position all the way. A1 continues the drive and I anticipate a PC foul. At the last second, A1 makes a spectacular, Kobe Bryant like spin move and fools B1. B1 lunges to try to maintain his position, but is clearly late to the spot and commits a blocking foul. I ignore the travel, like they do on tv, and blow the whistle to reward the kid for his Sportscenter worthy move. Unfortunately, my hand failed to get the memo, and attaches itself to the back of my head. My partner is a better official than me, and whistles the foul and makes the correct signal. Now, according to the masses, we must enforce a double foul, because we are obligated by this preliminary signal, which in this case was bogus.