In IELTS exam, reading and listening modules involve CLERICAL SYSTEM OF EVALUATION so if you get the answers of questions correct, you will get the EXACT SCORE. Hence it is easy to calculate the score that you could get provided you are sure you did not make any spelling and grammatical errors.
The evaluation of writing and speaking modules works DIFFERENTLY. These are evaluated by the examiner and the evaluation largely depends on the judgement of the examiner.
There are multiple factors involved that enable an examiner to decide what band he should give to you in these modules:
The points covered are RELEVANT to the task or the topic asked.
The arguments are CONVINCING and support the STAND TAKEN.
The examples covered are RELEVANT to the argument
Have you covered the points that HE IS EXPECTING on the topic?
Remember you may write very well or speak fluently in English, but if it is not in line with WHAT IS EXPECTED, how much ever hard you try, you will not achieve the score that you are aiming for.
It is important to think from an examiner’s perspective, how he would address the topic and what he expects, so that your STAND and ARGUMENTS are CONVINCING ENOUGH to IMPRESS the examiner to give you your desired band.