How do assessors choose between candidates?
It is important to remember that this will vary from company to company, but most companies will have an assessment criteria / score card where they give you marks for each task. Similar to how your academics mark your essays, they will be looking for you to show particular competencies in line with those on their job description.
At each stage of the process, they will give you a score for how you did in that particular section.
What is good about this is that even if you don’t perform well in one task (eg "I think I messed up that written exercise!"), the scores will usually be averaged out from your other assessments.
They are looking for people who can perform well overall. Don't assume you have not got the job because of one mistake - remember that they are looking for your working style and personality as well.