If the catch is routine, or if the no-catch is that obvious, then giving the catch/no-catch mechanic right away is not really necessary. You only need to give the immediate mechanic on a trouble ball, and in those cases, umpires rarely give both signals.
In other words, if a sinking line drive down the rightfield line is:
1. Caught in fair or foul territory, the umpire will almost always just come up with the Catch signal, and not bother signalling fair or foul
2. Trapped in fair territory, the umpire will almost always just come up with an emphatic fair mechanic
3. Trapped in foul territory, the umpire will almost always just come up with an emphatic foul call/mechanic
The only time I ever see what you describe (fair/foul signal, then catch signal) is on the routine catch. And I can only assume the signal is given to inform the scorer whether to write "F9" or "FF9" in the book.
