Am booking a reward flight OSL-LHR-OSL. With BA, the taxes come to $128.70, whereas with SAS the taxes are $70.70. Why such a difference in taxes for flights departing and arriving at the same airport? From OSL, both airlines depart from the same terminal (given the fact there's only one international terminal), whereas at LHR, SAS operates out of Terminal 3, and BA Terminal 5. Is that the difference?

SAS provides a handy breakdown of the taxes they charge:
Passenger Charge: 18.90 USD
Passenger Service Charge: 35.04 USD
Air Passenger Duty (APD): 16.76 USD
Total: 70.70 USD

As far as I can see, no such breakdown is available on BA.com

Any ideas why flying BA costs $50 more in taxes?