Any difference you are seeing is a typo or a mistake of some other kind. The amount of hours that you can record on the TiVo Series 3, HD and Premiere are identical given the same source. However, any hour estimate is only that, a rough estimate. Different channels are compressed at different bitrates and resolutions and that directly impacts the amount of hours that are available to you.
As a rough guide, a 320GB hard drive will net you 45/400 HD/SD hours. Moving up to a 500GB hard drive nets you 75/650 HD/SD hours, everything after that is linear. So a 1TB hard drive will get you double that, 150/1350 HD/SD hours and a 2TB hard drive will double that and put you at 300/2700 HD/SD.