Originally Posted by CPH-Flyer

SK may going by the IATA published distances by city pairs. These are not exactly equivalent to the great circle mapping for the city pairs. JAL uses the IATA distances, and as a result Tokyo to London is a longer flight than Tokyo to Paris, and both of them (or all four of them as Tokyo can mean both Haneda and Narita) are longer than the great circle track. I would imagine that as the IATA numbers are an industry standard that would be accepted as a calculation base if that is where the problem come from.

All that being said, I would query SK why they set ARN to LHR as a category two flight if it was me.