Why pick a different domain name for asset_host - and not just subdomains

Some sites, e.g. Github.com, uses subdomain for images & assets

assets from subdomain

Whereas others, e.g. Yahoo.com, uses a different domain name.

assets from different domain

Because cookie bandwidth does add up.

PS: I spot George Oat's picture on Flickr frontpage still. I wonders if it is a form of respect or neglect.