one way flights

looking to go to europe in late septemeber/ early october. would like to take the direct on condor to frankfurt on sept 24th but dont want to fly condor back because a timing issue. when i look to book a round trip the departure flight is only 269, but when i change to one way it jumps to over 400 bucks. same goes on other airline sites when looking at options for return, where a one way from london or paris to MSY is listed as more than buying a round trip ticket. any insight into why this happens? Ive used kayak , google flights and the delta website and it seems to be the same regardless of which search program i use. any help is appreciated