John, I don't think you misunderstood; it's just that either your manual contains a misprint (possible, but unlikely) or it is not using the same definition as some of us use.
What I think of as a "dispatch interval" is the amount of time from the moment that one train leaves the station until the next train leaves the station. If we adopt some terms from mass transit, then you can also refer to "dwell time", or the time for load, unload, and idle...from the time the train stops in the station until it starts moving again; and to "headway" which is the time between trains. I've rarely, if ever, seen those measurements discussed in a coaster context.
One exception was when I was trying to figure out the terrible dispatch intervals at Another Park, when I realized that they were doing an awesome job at loading the trains and keeping the 'dwell time' to a minimum (that's the part the crew has some control over) but that the dispatch intervals were disgraceful because the station sat empty for up to two minutes between trains. Mind you, this is with a train stacked on the block brake behind the station ("readies", I think you call them.).
--Dave Althoff, Jr.
Would it even be possible to calculate this at SFO, when sometimes they stack trains for minutes on end to flirt with each other, and sometimes they get the trains out as fast as CP?