Bob
Because of this exception, we see the 'ampacity' number from the individual breakers add up to more than the rating of the conductor ampacity that usually feed the panelboard they are installed on.
Again it is the actual or calculated load that is important.
In some residential 200 amp panels, you may see 60 or seventy breakers that add up to 700 amps or more. The actual load may only be 100 or so amps.
And yes you mention that additional installations are added all of the time without regard for the load. In my experience the average 'installer' does not know how to figure this out and will either ignore it or tell people to increase the service size, even if it is unnecessary.
The typical statement "I do this all the time".
Pierre
