Oh, that can be explained too! A memory charm wipes out the memory. If Hermione had done that to her parents, there would have been no way for her to life the enchantment and her parents would be forever thinking that they had no daughter . Hermione used some other type of magic that confunded her parents into thinking that they were different people. A memory charm can't do that--it only removes memories. So Hermione really never had performed a memory charm before.