Space Shuttle Coumbia: STS and Mission Numbers

This Columbia flight was STS-107. It was the 113th flight of the space shuttles.

Why don’t the numbers match? Why are there six more flights than mission numbers?

The STS numbers are assigned in the order that the missions are scheduled. If there is a delay of a mission for some reason, that mission number is retained even if it causes a situaiton where flight 45 comes after flight 48 or whatever.