It's not just a one way issue, it's a country of origin issue.
RT LAX-LHR is much more YQ than RT LHR-LAX.
Also, has nothing to do with distance as you can find ex US trips transiting through LHR being more than double YQ for LHR-SYD even though they are a shorter distance.
http://www.flyertalk.com/forum/briti...e-dollars.html