There are a few things to add in here (IANAL, but I'm close to quite a few).
A major element is billable hours.
When you want to reduce a bill for a client, during bad times or whatever, you can always under-bill the number of hours spent. Or change the mix of high-low-rate hours expended. Reducing the number of hours billed doesn't (necessarily) reduce the number of billable hours you can credit the attorneys.
While you can have equal salaries across departments, you have bonuses, which depend on how many hours you billed. Also, #billable hours is a great sort criteria for who's first on the shit-list and first on the promote-list.
A major element is billable hours.
When you want to reduce a bill for a client, during bad times or whatever, you can always under-bill the number of hours spent. Or change the mix of high-low-rate hours expended. Reducing the number of hours billed doesn't (necessarily) reduce the number of billable hours you can credit the attorneys.
While you can have equal salaries across departments, you have bonuses, which depend on how many hours you billed. Also, #billable hours is a great sort criteria for who's first on the shit-list and first on the promote-list.