Buying a flat: Carpet Area vs Saleable Area

Hello all, I am planning to buy a flat in hyderabad and noticed many builders are using saleable area which is 1.3 times of carpet area of a flat to sell to customers. As per my understanding, as per RERA builders are required to sell based on carpet area isnt it? What am I getting wrong? What can I do to get a fair price? Is it a common practice in hyderabad to cheat customers who are planning to buy flats like this? Any insights would be helpful, thank you