Why are guidebooks one way only?

While preparing for the first stage of this End-to-End, I decided to purchase the National Trail Guide for the North Downs Way. What struck me was that it assumes that you are always going to start from Farnham! No reverse directions from Dover, even though the plaque at Dover's Esplanade has a Start AND a Finish! 

This led me to do some research on the 'Net. Almost every site offers directions in one direction but not another. Do they assume we can decipher the instructions backwards, reading the guidebook like we used to read newspapers when I was a kid (ie, sports pages first!) or that we will always start at where the guidebook tells us to?

Hm.