Back when passengers could actually have interaction with the flight deck (cockpit visits), I was able to spend quite a bit of time in the best seat in the house. Generally, when the plane is realeased, the long distance flights do not have enough fuel to fly to the destination with required reserves for the FAA types AND a 5% fudge factor in case winds are greater than forecast. As they fly toward the destination, the amount of extra (5% of anticiapted need) decreases as the amount of fuel needed to complete the trip is less.

So, to try to uncomplicate this:
Say the fuel required by regulations HKG-YYZ is 100,000 pounds. Cathay loads an extra 5,000 lbs of fuel, just in case. By the time they get to Vancouver, they need only 40,000 pounds to get to the destination. Their 5% reserve factor abour federal requirements would be 2000 lbs. The aircraft can use the 3000 lbs reserve that is now available to complete the trip to the actual destination.

Hope this makes sense and helps.