I've been baking since I was about 7 years old (by myself since I was 9) and I still do very little experimentation. Yes, I'd know how to get a certain flavour if I didn't have the exact ingredients, but beyond that: no. I can make any recipe you put in front of me without trouble, and that makes me a good baker. But you don't get past that at high school. You have to become a chef or a trained baker to become expert. Snape is kind of like a chef. The Hogwarts students are just high school students. It's still all very basic - after all, they only do it for a couple of hours a week for 38 or so weeks a year. Not enough to become expert.

...

That was rather longer than I intended.