This sounds like a quirk of the website, where it gives you a slightly made up cost for each direction which is based on half the cheapest possible return on your outbound and return dates.

If you fiddle with the out and back dates and the classes, the prices can appear to jump and seem really cheap in one direction, meantime the other direction looks really expensive.
In actual fact the price changes are just glitches in the way the data is presented. You'll notice that the "fare code" being booked into is the same.

Once upon a time BA.com would occasionally suggest a sector would have a negative cost!