I'm pretty sure 1080i draws half the pixels in one pass then the other pass draws the other half alternating while 1080p draws the full picture in every pass. Is that not why 1080p uses twice the bandwidth of 1080i?
1080i
Most HDTV channels are broadcast in 1080i resolution. Programs broadcast in 1080i are sent at a ratio of 30 frames per second. Each frame has 1,808 vertical pixels interlaced with 1,920 horizontal pixels to create a continuous picture on your TV screen. Because 1080i is broadcast in an interlaced image, images such as a speeding car can blur when they move fast. The lines of the image are not rendered contiguously, which can cause a "messy" or blurred picture. It can also cause image skipping, where it looks like the image is delivered in stop-motion photography.
1080p
1080p HDTV resolution has the same number of pixels -- 1080i per frame -- but the image is sent at 60 frames per second instead of 30. The higher rate of frames per second in the progressive scanning of 1080p means that it takes up to twice the bandwidth as 1080i. Subsequently, many TV cable companies transmit or broadcast data in 1080i to prevent lag in the cable feed. This results in a lower resolution picture, but without the inconvenience of having your TV picture freeze or skip ahead without showing all of the program you are watching.