Presumably, the website is a general point-of-contact for the parent corporation, while the offers might be specific to your region. If it were a grocer or a pizza place, the deals specifically for you would likely be behind a "find your store" form. But Comcast likely wants to pretend they don't discriminate by region, so they just don't expose that at all.

They actually have distinctly different promotional offers in the same areas online, in person, and over the phone. You cannot register for an online-only deal in person or over the phone. It's a wonder their service even works at all.