Multi-segment itineraries between USA and UK (originating in either the US or UK) are cheaper bought in £ as only one US Passenger Facility Charge is applied, while when bought in $ there are multiple charges.

I have often wondered why fares bought in £ are slightly cheaper, and some investigation on ITA showed why. For example flying LAS-LHR return (or LHR-LAS return) on for example either AA or DL the US Passenger Facility Charge is $13.50 on a $ fare but only $4.50 on a £ fare (all other fare components are identical). (I verified that, as one would expect, the fares charged by aa.com are identical to those quoted by ITA.) The size of the difference increases with additional segments.

Is this a glitch, or is there something in the rules for US Passenger Facility Charge? Maybe the explanation is somewhere in the 200 page FAA regulations?

For US residents with a credit card that does not have a foreign exchange loading, it may be cheaper to buy a multi-segment itinerary in £. For UK residents it is an incentive to pay in £ (although there is sometimes availability in cheaper fare classes in the US market).

[For those not familiar with ITA, you can specify sales city as LHR to get £ fares and BOS to get $ fares. Similarly on aa.com you can specify country of residence as USA or UK.]