Question

Why is the distance from a planet to the sun an average distance?

Answer 1

All of the planets circle the sun in elliptical orbits.

Explanation:

The figure below shows the orbits of the planets.

Answer 2

I would say because the planets descibe an ellipse which doesn't have only one radius.

Explanation:

Considering the orbits for the planets of our solar system:

you can see these elliptic orbits that are contemplated also in Kepler's first law.
As such orbits/planets have a minimum and maximum radius/distance from the Sun. Normally you can apprximate the orbits as circles (for some simple calculations) introducing an average radius.