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.
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.