It's an Apple thing.

Many older typewriters had no bump on ANY key. Even the pre-ADB Apple keyboards are missing the bump. It was with the ADB era (and the //c, for that matter,) that Apple started putting a bump on. I don't know why they chose D and K, but that became the "Apple standard" up until the USB models. I have one of each ADB Apple keyboard model, and they all have it on D and K. (Again, plus the //c and //c+; I don't know about the //e line.)