My teacher does this sometimes(esp. with Xiao Baji Quan). Sometimes it's because he learned it different ways from different teachers, so he memorized the variations. I also think he teaches some forms a bit different to each of us, depending on our previous martial arts background or different attributes.Hmm, I often wondered why the forms were changed often and without any explanation. At first, it was quite frustrating, but just had to learn to except. Now I have learned that Shifu`s add and subtract "stuff" in forms. This for many reasons. Not much time right now to get into.