If you use all the lye necessary to exactly saponify all of the fatty acids in the oils, the soap would have no excess lye or excess oil (next to impossible without a chemistry lab). It would have a zero discount and there would be no superfat. If you discounted the lye, let’s say by 5%, 5% of the soap would contain oils that have not chemically reacted with the lye. i.e. all the lye was used up in reacting with 95% of the oils. The soap would have a 5% superfat. These unspecified oils are good for conditioning the skin. A soap with no discount or superfat will clean better. However, other than conditioning the skin, there are other important reasons to discount the lye (or superfat the soap). Please see the next question.
The SAP values used to calculate the amount of lye – in this calculator and in others – is an average. If you purchase oils/fats that have been assayed by a chemical lab, you will know the exact SAP value of the oils/fats. (Your wallet will be a little thinner too). If you weigh these oils and the lye on very precise laboratory scales, you can confidently make a true zero discount soap.
One of my references gives the Potassium Hydroxide (chemical symbol KOH) SAP value for coconut oil as 250-264. The SAP for other oils is specified similarly as a range of values. Why? There are many variables that contribute to the chemical qualities of a carrier oil. Here are a few to give you the general idea:
Species of the botanical used to make the oil.
Geographical location of the source botanical. i.e. variable climate.
Varying types of processes used to create the oil.
Seasonal differences in when the botanical was harvested.
Maturity of the botanical when it was harvested.
The minimum 5% discount/superfat is recommended to allow for these variances so you do not end up with soap that contains unused lye.