Total distance per day for different travel paces?


The travel pace description and table on page 181-182 of the Player’s Handbook states that a normal travel day may contain 8 hours, and gives the following table:

 Pace:   /Hour    /Day         Miles in an 8-hour walking day:   Fast   4 miles  30 miles --> 4x8 = 32 miles (-2 miles a day) Normal   3 miles  24 miles --> 3x8 = 24 miles (OK)   Slow   2 miles  18 miles --> 2x8 = 16 miles (+2 miles a day) 

Why is there an difference of 2 miles for fast and slow pace?