Can't say I've seen many cases where a roundtrip costs more than two one-ways for the exact same flights (which I believe is what you are claiming). In many cases, it is often the opposite (as you will often see cheap fares that require a RT purchase). I suppose the exception would be Canada flights where you can benefit from the exchange rate by purchasing the return in CA dollars. For domestic flights, I would very much doubt this is ever true. Care to post an example?
Try it with an open-jaw international-
Oct 31 SFO-SIN one-way $449
Nov 11 AKL-SFO one-way $642
Open-jaw r/t (same ticket) $3570 on UA's website (obviously booking into a different fare bucket on the return) but even if you use itamatrix, you still get a higher fare (but much closer) at $1272.