Fuel surcharges have nothing to do with fuel. Guava has it about right. In the US, fuel surcharges on a ticket for the same flight are usually more if you buy the ticket last minute than if you buy in advance. When AC had a fuel surcharge in place in North America, I paid a higher fuel surcharge going from EWR-YYZ then someone going YYZ-YVR.

These are good examples - in # 1 - how does the time of ticket purcahse increase the amount of fuel costs per person, and in # 2 - why did I pay a higher surcharge for a flight of 1.5 hours vs. one of 5 hours. Certainly, it had nothing to do with fuel. All to do with competition.

Ironically, this is the same reason why I can also get a cheaper ticket from the US-Europe via YYZ with Air Canada then I can on the same route but originating in YYZ. I suggest you check the tickets you were looking at on AirCanada.com, and I suspect you'll find the same thing.