It's mainly specialties, not just levels. Are you a graphic designer that can also build your designs for the web? Were you previously consulting for our largest customer? Do you speak Welsh for our important client in Pembroke? Did you previously work in tech stack X and want to get back to it, and could lead our effort into that market? I'll pay more for these even if they come in at the same grade. Pay is very subjective, it is a prediction about the value an employee can or does bring balanced against their next best option.