That is a good question. I've talked to many native speakers who cannot understand why they did not get the score they expected, and I'll go through some of those reasons. One, some native speakers think their handwriting is good and it isn't. That's a problem if you're using a pencil and paper too. Some native speakers think that their spelling is good and it isn't. A lot of native speakers don't understand how to use paragraphs very well or punctuation. That's a problem. But the big one, I think, is that native speakers think that if they write, simply write a coherent essay, it doesn't really matter that they answer the question fully, and they need to answer the question fully in order to get a good score in that first criterion. So if you only get a five or six for that first criterion, and the other ones eight and nine, then you're only going to end up with something like a band seven, where you really should be getting an eight or nine.