I don't fly overseas much, but on those times I've had cause to look at overseas fares, RT is often much cheaper than two one-ways. Sometimes cheaper than one one-way.

Airfares have gone through many permutations. I was only 21 when deregulation took place in 1978, so I had flown very very little, but I think it was pretty much just Y and F and that was it. Following deregulation, the first change I remember was "super-saver" fares which required a 30-day advance purchase, 7 day minimum stay, and were 40% off. There was a time (I think the early 1990's) when round-trips were much much cheaper but required a Saturday night stayover. Lately I've noticed that fares vary most by day of the week (Tuesdays and Wednesdays are cheapest).

I checked American Express and they had the same discounts (10% on F, 5% on Y) compared to the AA site, for both OW and RT.