This question was asked by one of my students the other day and I have to admit, it somehow caught me by surprise! The common misconception is that examiners assess IELTS writing and speaking by comparing performances of various candidates. This is not true!
IELTS examiners grade your writing and speaking performance against a set of criteria. If all the requirements for that band score are met, then the candidate will be awarded that score. Assessors do not look at different papers or reflect on other candidates’ speaking to base a judgement on your score.
So, having said this, IELTS candidates should strive to meet these requirements by working towards improving their language skills and practising as much as possible. Try to emulate how fluent speakers or writers express themselves and always aim high. Do not settle for just getting a Band 6 or 7. If you believe you have what it takes to achieve a Band 9, then work towards it.
I hope this clears up any confusion about how you are assessed.
Until next time!