Another advantage to superfatting soap is that it gives you a little wiggle room with oils that have varying saponification values - since it's better to have a little extra oil than extra lye.
The traditional way to super fat a soap is to reduce the amount of lye called for in the recipe by up to 10% to achieve a desired result. One major problem with high levels of super fatting is oils going rancid in the bar, and the associated odor.
At Voyageur we use a different technique - we use only moderate super fatting, and combine it with the use of anti-oxidants to maximize the shelf life of superfatted soap.
Instead of just reducing the amount of lye, we add 2% of a premium skin care specialty oil that is reinforced with Vitamin E w hen our soap is brought to a trace (and the molecules of oil and lye are marrying up.) This way, only your specialty oil is left unmarried in the soap in its natural state, rather than a mix of all of the carrier oils you'd have by just reducing the lye in your recipe.