There are two different meanings to the word "superfat". The one you are using seems to be setting aside a special oil to add at the very end. It sounds like you did not include that oil in your calculations?
The second meaning (the one the
lye calculator is using) is simply what percentage of oil you have that is more than the amount of lye will saponify, so there is extra fat (aka superfat) left over in the finished bar.
So for the same amount of oils, you will use less lye if you want to have some of the oils not saponified in the finished bar. So if you specify a 10% superfat, that means you will use less lye than for a 5% superfat.