A Captain is regarded as equivalent to a grade 10 manager by HR, for the purposes of benefits, but earns considerably more (about the same as a VP). A First Officer is regarded as equivalent to a grade 9 for benefits, but again earns more than a Grade 10.
Its just the ground weenies way of keeping us in our place, even though HR knows they have to give us a somewhat competitive wage, they do not give us the same benefits as a manager that earns the same as us.
And this company sure loves it grades.