Today, Mercury is as close as it will ever to from the Sun thanks to the fact that it is at perihelion, a point in its orbit that is closest to Sun.
What many people may not realize is the fact that Mercury (and all other celestial bodies) do not orbit their parent bodies in circles, but ellipses, which are slightly elongated circles. Result: any given day of an orbital period, any orbiting body will be at a slightly different distance from its parent body. As for those planetary distances taught in schools, they're the planet's average distance from the Sun.
Additionally, Mercury is as fast as it will get today, too. Why is this? As a planet locked in an orbit gets closer to the Sun, the Sun's gravitational force on the planet increases and the planet will speed up. Then, as the planet rounds the perihelion point and begins to move farther away from the Sun, its speed will decrease as the distance to the Sun increases, culminating at minimum speed at, you guessed it, aphelion. This fact was first discovered in the early 1600s by Johannes Kepler and serves as his 2nd law of planetary motion.