You’re basically competing with the whole world (yes that’s how harsh they grade). Every subject has a mark scheme where the reason why you receive each point is explained.
If let’s say that a test is out of 90, but this test was hard so some of the highest points are around 80, than the boundaries for 7 would be between 80-90 points. If the test was easier, let’s say the highest points around around 88, than the boundaries for a 7 would be 85-90. Basically only the best of the best get a 7 :”)
Just do your best. Look at past exams and how they’re graded so when you do your test you have a clear idea on how you will be graded