I just had to book a circle trip on UA - DCA/MCI, MCI/ABQ, ABQ/DCA. Lowest one-way fares on the three legs for the flights I need are $109.60 (G class), $142.60 (L class), and $186.60 (L class) respectively, adding up to $438.80. However, if I book the three legs in the same PNR, the lowest fare I get is $486.20 using L, T, and L class repectively. I called UA and they confirmed that the above is correct, because the one-way G fares between DCA and MCI are not combinable on a single ticket with one-way fares to any other city pair. They told me I could book the lower G fare, but only if I booked each leg in a different PNR.

When I looked up the rules on each of this unusual G fare, I found a specific statement in the rule as follows --

"Multi-component circle trips not permitted".

Again, when I spoke with UA, they said this has come up on a number of fares for certain routes, but rationale is not clear. They said these special one way fares are designed to compete with certain ultra-low fare carriers and I emphasize, they said there is nothing wrong with booking the one way fares as long as I do it in separate PNRs.

Just out of curiosity, I tried booking a roundtrip DCA-MCI-DCA on the lower G fare and that worked fine!

The airline explanation - you can't book the leg from MCI to ABQ on the same Ultra-low fare carrier as the one from DCA to MCI. So if you want a single ticket for your entire itinerary, the low fare carrier is no longer viewed as competitor for UA and they don't have to match their fare their fare from DCA to MCI. Since you are allowed to book the trip in separate PNRS, this seems to accomplish nothing for UA other than to create more work for everyone.