The obvious answer to this question is that it depends on the time of the year. These exams are carried out yearly with application deadlines varying.
With the exams being carried out on an annual basis the cut off dates for registration vary slightly although they keep to a general pattern of having to register by the end of September in order to sit the exams in November or December. By creating and sticking closely to this routine it allows students and colleges to plan their coursework ahead of the exam.
When you decide to take an exam it shouldn't be a spur of the moment or last minute decision. You should be planning in advance aware of the timetable so that you can devote the proper time to your study. There is a lot more involved than simply sitting the exam, you should have some sort of study course planned in advance either in a school or college or a dedicated program of home study ring-fenced.
Although the actual questions are changed from year to year the questions have to be of a similar standard year by year in order to provide fair comparisons. If the questions were of a lower standard one year then it would be unfair on people who had sat the exam in other years.
By maintaining a similar standard it is fair to compare the results of someone sitting the exam one year with people who have sat it in other years. This allows prospective employers to make accurate comparisons between candidates for jobs knowing that they are comparing like with like despite having sat the exams in different years.