There are several factors involved. The difference can be anything from fairly small — say 10% — to quite large; maybe 50%? When it’s larger, the reason is generally one of fare class. For example, take AUS-DTW as an example. Even though there is significant competition on this route — DL flies it nonstop, and I think NK still does; WN also has quite a bit of lift into DTW — it’s quite common for the lowest one-way fare to be an S or a T while the lowest round-trip fare is an L or a K. If there’s no L or K inventory, you probably wouldn’t notice. If there is, the difference is substantial — hundreds of dollars on the round-trip. (At the moment, it seems they’ve raised prices — the lowest roundtrip is an S and the lowest one-way is a W).

Because of the way that UA generates refundable and domestic first-class fares from the underlying discount economy fares, much of this is true for those fares; it depends upon whether or not the particular fare was one that RM chose to pair up with a refundable / first class fare. I haven’t figured out if there’s a pattern where they will and will not.

It’s more prevalent for smaller cities without LCC competition, and it’s something that AA, DL, and UA all do. If most of your flying is from a UA hub to a large city, you may not see this — for example, AUS-SFO appears to price out as two one-ways. However, being at a hub doesn’t make you immune — SFO-OTH (North Bend / Coos Bay, OR) shows this exact same pattern.

The overall TL/DR is that airfare pricing is extremely complicated, to come to FlyerTalk if you need help deciphering the fare rules, and to try both one-way and round-trip pricing, particularly for unfamiliar markets.