When hunters awaken, they got ability and a rank and they can't change it (except for jin woo and double-awakening but it's very rare)
The rank go from E (the weakest) to A (strongest) and then you have the S-class, you are s class when your strenght can't be measured so there are big difference between s rank, some are just little higher than A rank when some are as strong as the army of a country