Creating a piece just for me is waaaaaaaaaaaaaay easier than creating a design for a class. When I work just for me I can use any threads I think will work, I can create stitches I could never explain in words, let alone in writing.
When a design is created for a class it has to be explainable on paper. It has to be demonstrable either on a stitching frame or on a white board -- or in my case with a Power Point presentation. So certain stitches or techniques aren't used because it is too hard to explain them.
Also, when creating a class project only materials that are readily available can be used. Using a discontinued thread or some of the overdyed "accidents" that dyers make available won't work as the teacher has to keep in mind what will be available to put in kits. Also, the teacher has to keep in mind the cost of certain threads -- a class with a too expensive kit will never sell.