The 1 way fare you are quoting is a fare that has "H" availability and only a 7 day advanced purchase requirement. Your roun trip fare has only "t" availaibilty (which is almost sold out, so jump on it now) and requires 21 days advanced purchase. So in a day or two, if 2 more people buy and inventory bucketsdon't change, that cheap roundtrip fare will be sold out and the cost will go up, and 20 days before departure, even if T is still available, you won't qualify for that fare, and the price will go up. Most likely you would still beable to buy the same 1way farethen, most likely, even up and until 7 days befor departure, unless demand is very high.

So for the reasons others have mentiond, the other reason is the 1 way fare is available on short noticeand on much fuller flights that the highly restrictive round trip fare. Both of those are things that add value to a buyer, within 21 days and on full flights. As such, they command a premium.