I expect the difference is due to the fact that your first flight determines which airline will ticket the whole itinerary, and some airlines add fuel surcharges while others don't.
Your westbound (cheaper) itin starts with an LA flight which, I think, will be ticketed by AA, and they do not add fuel surcharges.
Your eastbound (expensive) itin starts with an IB flight which, I think, will be ticketed by IB, and they do add fuel surcharges.

You can check the exact taxes, fees, and surcharges, by clicking on the + sign beside the final price, after you have been transferred to the ticketing site, and before you enter your credit card info.

Be aware that there is a bug in the system at the final stage after you enter your credit card info. You may get an error message (15012) especially when ticketing with AA. It is not easy to resolve that error as even the oneworld IT department doesn't know what causes it.

Good luck!