Is the distribution of ability levels really a bell curve? I would expect it to be roughly trimodal, with a large group at "generally competent", a small but sizable group at "incompetent and painful to work with", and another small group at "creative innovator that raises the productivity of everyone around them".

I'd also expect that everybody wants to think of themselves as being in the last group, and so the companies all advertise for that. But realistically, very few people actually want to hire creative innovators and even fewer still will listen to them once they do, so companies are really gunning for that middle group. It's quite possible to parcel out that middle group into 5% chunks.