I think it's because the professor was off to the side, drawing on a tablet, while we were staring at the screen it was projecting on. It also didn't help that we were in a computer lab, with huge 22 inch iMacs in front of us, making it hard to see the screen and basically impossible to concentrate. When a professor writes on chalkboard, your eyes tend to follow them around the room, rather than staring at what they wrote.