Extremely lucky or perhaps you just don't realize that it's actually not right
The problem with pre-slotted nuts is that when they slot them at the factory or wherever it's done they come to you with nice rounded nut slots that fit the strings well. So you don't want to touch those and as such the only way to get the height correct is to sand at the bottom. That's simply not the way it's done and you will never, outside of extreme luck, get all the slots right that way.