Do all grade boundaries depend on how the rest of the country performed on the exam?

Or are there specific guide lines for some?

That is actually the most common misconception, the grade boundaries depend on how difficult the exam is determined by the exam board, who do not actually just use percentiles, but there own forms of judging difficulty.

This is so that improved teaching, or differences in any one year does not leave identical candidates with different grades. This is so that GCSE's can be timeless qualifications.