Each airline files their carrier overcharges ("fuel surcharges") in the GDS. I am not aware of any handy tool to see this, although matrix shows it but only for "normal" itineraries, not RTWs. Many airlines charge per segment for their own flights, while some impose an additional surcharge over the whole trip. In my experience, AA charges all carrier overcharges, while some airlines (e.g., JL) seem to not charge quite as many. In addition, I think (but am not sure) that some airlines impose their own overcharges for the whole trip when booked through them. So, AA is in the middle. Booking with JL may save several hundred USD, and booking with BA may cost a few hundred more.