why the numbering of atoms in purines and pyrimidines is different?

Question

numbering of carbon atoms in purines and pyrimidines is different why?what is the significance of such change in numbering?

Answer 1

The simple answer is because that's how the International Union of Pure and Applied Chemists (IUPAC) decided to number them. In general, ring atoms are numbered so that the nitrogens, in this case, get the lowest possible numbers.