Well, everyone's hair look is not always exactly the same way, it depends on its lenght, how it's dryed (if with an hairdryer or just in the air), the haircut, even on the shampoo that's used to wash it.
Off course for Dylan was the same, and I can imagine him being very casual about his hair care, so he doesn't appears always in just one way over the time: sometimes is more curly-haired, other times he seem almost straight-haired. I don't think that he was using a straightener.