Your intent is to purchase a roundtrip. The reservation system has a mandate to offer you the lowest fare for the itinerary you specify. If the lowest roundtrip fare isn't actually the lowest fare available, you'd observe an unexpected fare calculation. Most people would probably see this as good and desirable (they like saving money).

A-B-A is a round trip; A-B-C-A is a circle trip. More precise definitions of "round trip" and "circle trip" appear in United's Contract of Carriage.

An "end-on-end combination" involves adding two different fares at a "fare construction point." Think of it as an opportunity for a "free" stopover, if you prefer.

End-on-end combinations don't just affect pricing; the fare rules of each component apply. For example, I've ticketed SFO-IAD-TPA-ORD-LGA as an end-on-end through Tampa: that sequence could not have been sold as a through fare.