The technique specified in the Flight Manual will be based upon testing, and the optimum results being obtained with "average" pilot skill. There are differences in 172's over the years, including installed power. This can result in differing optimum techniques. Obviously to get the best results, you should fly it the way the Flight Manual says. This is not to say that cannot get even better results, but in doing so, you might be sacrificing safety in the case of an unexpected event.
Every aircraft manufacturer wants you to fly safely above all. They will write down the techniques and performance a little, if doing so more assures safety, compliance, and keeps them out of court!