To your point below, I suspect the 8-10 year person is creating more value with less supervision than a new hire. Being able to manage a project/client solo, even a small one, makes your work incremental revenue to the firm. More so if you're involved in bringing in the business....
I have new hires; they take a LOT more supervision... and our group's output is limited to the speed at which our senior folks can originate projects...