Originally Posted by Kiwi Flyer

That isn't quite right. The lower rates (labelled economy above) apply to the main cabin, and the higher rates (labelled business and first class above) apply to all higher cabins. So premium economy gets charged the higher rate, whilst all-business flights get charged at the lower rate. There are also rules about transits (some, but not all, transits will not attract the charges) and treatment of trips UK-europe-intercontinental.

Any way, to answer the question - it is different because of UK tax regulations.