Is it possible that the difference I'm noticing has a mechanical componant? It sure feels as though less pressure is required with the external controller than with the on camera. I know that in shooting with firearms a lighter pressure trigger helps to increase accuracy by reducing muscle tension. Perhaps the lighter touch is enabling me to initiate the zoom with finer control?

Or course what you're suggesting is possible for your particular finger-feel. It may be that you personally get more "feedback" from the external controller than the built-in camera rocker. But the actual speeds that are available from the camera don't physically change; as you suggest you might just have a more natural interface with external controls.