Tire degradation (or more accurately, the wear rate) isn't linear. A full tank of fuel is harder on the tires than a light load. Assuming you don't change push levels throughout a stint (which is generally faster over a race), you might use 8% of the life on the first lap after stopping, and 3% on the lap before you stop. It starts high and tapers off as the fuel is consumed (which makes the car lighter).
Not only fuel, but tires are made of layers, which are of different durability. The last layers are more durable, to prevent tires to explode or shred, but they provide less grip.
