I wonder if what's going on is that most of the work is done by people who joined within the last two years. Or maybe somebody leaving and having to be replace

I wonder if what's going on is that most of the work is done by people who joined within the last two years. Or maybe somebody leaving and having to be replaced by a new hire kind of rolls the dice on headcount, which low and mid-level managers might find interesting. The answer I like the most is that, as the article says, when you hire a new person you have to pay, on average, market rate. When you're evaluating whether to change the pay of Joe Lifer, you sometimes have to pay market rate but often you don't have to. Maybe you'd save money by giving raises to Joe Lifer, but who knows? You're hiring people anyway, so if Joe Lifer wants to leave, that's a shame, pay the new guy instead of him.