The 1798 (WLJBPAU1) fare has a restriction on the outbound, it is valid when outbound is one of the following.
EK FLIGHT 433
EK FLIGHT 409
EK FLIGHT 415
EK FLIGHT 425.
The reason you might be getting the higher fare is because the fare bucket W may be sold out on one or some of the flights. If you are flexible with dates have a look at some of the before and after dates of your planned trip.