This can happen quite frequently on international routes. (By now you should know that international fare construction is anything but logical and comprehensible.)

Usually when the fare structure is asymmetrical, meaning fares ex- the origin are more expensive than fares ex- the destination, you can get cheaper fares by pricing as two one-ways vs. a round trip. A good travel agent would know to always check this.

JFK-TPE on CX is a good example. Ex-JFK there are only full J and F fares. Even though the one-way fares are more than 1/2 the r/t fares, it's cheaper to price the return as a one-way ex-TPE where there are A and D fares. (And even the F and J fares ex-TPE are cheap enough to make up for the higher-priced o/w fares from JFK.