It's funny how it seems that the butter doesn't matter, but it sort of does, also! Here's an example of something related: if you have six pieces of bread and two meat burgers, how many hamburgers should you make? If you make only two hamburgers, you will have two pieces of bread left over. We don't want to have any food leftover, though. That's why we should make three burgers -- it's okay to add another meat patty but not okay to have left over bread.
This is why Professor Loh calculates the number of recipe's worth of flour, sugar, eggs and butter he has, separately. Then he will figure out what he has the "most" of (like how we had more bread than meat burgers in our example), and make that number of recipes of cookie.
So since the butter amount we started with was zero, that means we don't bother calculating how many recipe's worth of butter we have, because we know we just have to add as much butter as we need to make however many recipe's worth of cookies we need.
I hope that helps to answer your question! Good luck with the rest of your learning and exploration of math!
Happy Learning,
The Daily Challenge Team