Planning a trip to Paris next summer and have the outbound flights reserved. However I am having an issue getting the return (CDG to LAX) because BA has not released award flights past July 18. So as an option, was looking at booking LHR-LAX now on AA and then booking a CDG-LHR later on using Avios points. That got me looking at the cost for the flights. A strange thing popped up. If I book a LHR-LAX flight, the taxes, on AA first is about $289 per person. However, if I look at the fees for CDG-LHR-LAX (obviously for an earlier date where flights are available) with the CDG-LHR being on BA and the LHR-LAX flights are on AA, the taxes REDUCE to about $202 per person. Anyone have any idea why ADDING a flight segment, on a different airline, would reduce the taxes?