Well, I suspect they're also "overhiring" a lot of the time -- bringing in people that are on the high end of the intelligence spectrum (if not experience) ...

Well, I suspect they're also "overhiring" a lot of the time -- bringing in people that are on the high end of the intelligence spectrum (if not experience) ... and then putting them in boring jobs. That's a sure-fire way to see a significant chunk of your work force churn every few years. And they may even be accounting for that -- this may only be half-way to prevent employee loss, but just as much on managing and understanding it.