cluelessPHP
Same reason why http://www.phpbuilder.com/ is not the same as http://board.phpbuilder.com/
You have one registered domain (gotsocial.co.uk), but there can be multiple hosts at that domain doing different things, each identified by a different host name (gotsocial.co.uk, www.gotsocial.co.uk, etc.gotsocial.uk…). Or to look at it another way, you've got a domain registered with Nominet UK, which identifies gotsocial.co.uk with your server, and your server subdivides the domain further by putting part of it under a www subdomain.