This is because British Airways adds significant fuel surcharges to their flight prices. (upgradedpoints.com)
In addition, flights operated by Air France will incur significant fuel surcharges on itineraries originating in either the U.S. or Paris. (upgradedpoints.com)
When you run your search on ANA's site, make sure you choose flights that are operated by United Airlines, since many other Star Alliance partners will require you to pay significant fuel surcharges. (upgradedpoints.com)