Here's why:
* Highly elliptical orbit: Mercury has a highly elliptical orbit, meaning it's significantly more oval-shaped than a perfect circle.
* Kepler's Laws: Kepler's Second Law of Planetary Motion states that a planet sweeps out equal areas in equal times. This means that when a planet is closer to the Sun, it moves faster, and when it's farther away, it moves slower.
* Mercury's eccentricity: Mercury's orbit has the highest eccentricity (a measure of how elliptical it is) of all the planets in our solar system. This high eccentricity leads to a greater difference between its fastest and slowest orbital speeds.
In short, because of its highly elliptical orbit, Mercury experiences a much larger variation in its speed as it orbits the Sun compared to other planets.
