One shoe is leading , ie the front edge moves outwards and contacts the drum , the other shoe is a trailing shoe. When properly adjusted the trailing end of the leading shoe is set so it is nearly in contact with the drum , so when the brakes are applied most of the shoe makes contact as the rotation of the drum moves the shoe towards the bottom adjuster and this pushes the trailing shoe so it also contacts the drum over most of its length.If the brakes have not been adjusted for a long time and high mileage you will see a difference in the thickness between one end and the other. I have seen differences of 0.5 to 1.0 mm on brakes that needed a lot of adjustment. After cleaning up the shoes and drum and backplate with a wire brush and then 120 grit emery I use brake cleaner to remove all the built up dust from the shoes and the drum,. don't forget to grease the hub shaft (do not grease the threads) before refitting the hub, and fit a new hub nut torqued to 290N.m for Alko 32mm AF hub nut, never reuse the original hub nut, its a one-use type.