Why is this?

[ACM]

Not that it really matters, but the majority of websites display the "www." before the domain name in a Google search so why doesn't mine?

http://img293.imageshack.us/img293/8585/acmwin.jpg

[alvo]

Because that's what the link was through which they found your site. In your screen shot, note that hotspotblog.com also doesn't have the "www"