In technical fields it's not uncommon for supervisors to earn less than those whose authority otherwise ranks below them.
It is basically just a question of market forces. If a particular position is sufficiently specialised when the supervisor role is more generalised then you might expect to pay the specialty more than the generalised role - even though the generalised role is more senior.
It is basically just a question of market forces. If a particular position is sufficiently specialised when the supervisor role is more generalised then you might expect to pay the specialty more than the generalised role - even though the generalised role is more senior.