Roll resistance. The composition of tires varies. Both the construction and rubber compound can effect how "difficult" it is to roll the tire.

Stickier tires are just that - stickier. This means the tread tries to remain atached to the road surface. Good for laterial control and traction but also increases the effort needed to roll.

This is greater at low speeds and less at high speeds. But low speeds were where you used to get your better mileage.