Can anyone explain what is happening with the fuel surcharges on one-way tickets?

I was looking at a one-way and returns to Calgary in CW.

A return to YYC has a fuel surcharge of £359 + £197 of other stuff

A one-way LHR-YYC has a fuel surcharge of £180 + £165 of other stuff, mainly APD

A one-way YYC-LHR has a fuel surcharge of £276 + £32 of other stuff

So ... you are basically paying a 'surcharge' on your existing surcharge of £97 per person, return, if you book 2 x one-ways instead of a return. Why?!

I appear to have accidentally paid this when booking my Mum's recent return, as I booked 2 one-ways in order to mop up miles in various accounts. I never assumed I was paying a fat penalty for the privilege.

It seems that the YQ is now assessed depending on primarily the point of origin and the market for which the fare is targeted, assessed at a level at which the relevant market can sustain instead of any logical basis.