The front and rear tires wear at different rates. Think about it. All that parallel parking. All those three-point turns. With each turn of the steering wheel, pressure is bought to bear on the front tires.
Resistance causes friction, which in turn produces heat.
The front tires wear quicker than the rears. There's a good reason for this. Tires develop wear patterns as they age. Some of these patterns are tied to the suspension system and the alignment. That's why we keep the tires on the same side of the car.