Well I know the BB Storm's viewing angle is way better than the iPhone. You have to look at pixel density and such. You can't just say WHY OH WHY and not really give specs of each device's display.
If the Bold is anything like the Storm then I'd say it was just a more superior screen.
Basically because they are the same resolution, but different sizes. The bold is packing the same amount of pixels into a smaller area. I owned one of those for a few days, the screen was awesome.