According to the IELTS examiners, it is the quality of your writing that makes that half band of difference. Let me explain. I have written a post explaining how your Writing task gets graded, what things are important and what you can get marks for. So let’s take for example one of categories there, “Selecting what information to present”. If the examiner sees that you attempted to do that, but didn’t do it quite right (for example threw too much information away) – that could mean that you get half of band for attempting but not the full band because you didn’t do a very good job selecting information. Usually messing up in just one category is not enough to cost you half-band; it would take two or three mess-ups of that kind to get the half-band taken away.
Another example – weak grammar and poor choice of words can make the difference between band 5.5 and 6. The work can be written in beautiful English but if it doesn’t follow the IELTS format, you’ll never get beyond Band 7.
So I think the bottom line is – they introduced the half bands to make difference between students that know what IELTS writing rules are, try hard but do a poor job following the rules and students that do a good job in fulfilling all the requirements.
