After writing more than 70 writing tasks and corrected by ex IELTS examiner and following all the IELTS instructions what more can you expect. My wife says that whenever she has used to write gibberish she has scored 7.5. and by the way she has attempted this might be 15th time. Yes for nearly 13 times she did it without any IELTS coaching nor going through any material. She has scored better in writing i.e. 7.5
I understand your frustration, however, please understand that "writing gibberish" can get easily 7.5 , why? Because it's a language exam, not a knowledge test.
She must've scored a higher score, because of the way she performed specifically her Task 2 (which carries almost 70% of your total score for the writing module)
Even, I was skeptical about the way IELTS judged my writing, but the module is actually hard, as it's "Academic".
You've got a really good score in all of the modules, In writing, I suspect, you're missing out on maybe layout,punctuation and the way you're ordering your ideas.
I cannot stress enough of how your Tasks have be extremely coherent, with appropriate vocabulary, you've have really pay a closer attention to the topic as well as straying away from it can make you lose a lot of points. Also, make sure you answer each part of the question with appropriate length.
It's like writing a dissertation in very less time.
Trust me, Academic writing is way more complicated than General Training. Even native speakers can score poor in this module.
Wait, are you talking about General IELTS or Academic IELTS?
I gave the exam a second time, and I was really convinced that IELTS is indeed a very accurate indicator of your performance. Which is why, it is used by so many English-speaking nations from issuing study permits, work to granting permanent residency.