Here is what the USGA says in its Handicap Manual:
A "Slope Rating" is the USGA's mark that indicates the measurement of the relative difficulty of a course for players who are not scratch golfers compared to the USGA Course Rating (e.g., compared to the difficulty of a course for scratch golfers. A Slope Rating is computed from the difference between the Bogey Rating and the USGA Course Rating. The lowest Slope Rating is 55 and the highest is 155. A golf course of standard playing difficulty has a Slope Rating of 113.
My question is, why is 113 the standard rating?