Exactly. People are not only travelling on one-segment juants.

OP, it takes a bit of reading up on how faring works to understand the genius in it. One of the purposes is to permit inventory and revenue to be managed by changing fares or changing availability, as appropriate to a given. Almost every airline uses this method to generate fares, and they do so because of the built-in flexibility. Leaving fare buckets open in some fare classes that do not have a point-to-point fare published is an important part of that system.

To take a quick example, United might have two flights from XXX-YYY on a given day, and might generally keep only a B fare published for that route, as there is a lot of demand and a near-monopoly on United's part. (United probably permits XXX-YYY-ZZZ as a routing on some liesure routes in G, as well, to get some of that traffic.) Now, let's say that United notices that it's underselling a few of the XXX-YYY planes a week, but that most are still filling up. Further, given the nature of the local traffic, the flights that are filled up get relatively full early on, so that there's G availability only on those flights that are ultimately going to go out less-than-full. What does United do to grab some more revenue from the less-than-full flights? Simply publish a new fare for XXX-YYY in G class, perhaps with certain restrictions (e.g., a minimum stay) to keep the folks who normally buy the B fares from mooching.