I purchased this book together with Common Mistakes at IELTS Intermediate. This book isn't what I expected *either*.
It is based on the analysis of exams scripts which scored band 7 or higher but the mistakes analyzed in this book are relatively simple (still!). The book deal with articles (!!!), when to use capital letters (!!!!), punctuation, using propositions, comparisons, contrasts, numbers and amounts, countables/uncountables, and a long list of commonly confused nouns to name the "best" part of it.
There is, indeed, a very interesting section on describing statistics.
If this is the kind of mistakes one can make and still get a band 7, I would be inclined to think that IELTS needs to be profoundly revised. As an alternative, I suggest the band description to be re-worded and made less intimidating and more in line with the actual performance that is expected from examinees.