font family issue

I found a website with the same theme as mine but using a different font family which I happen to like better.
However, it turns out it is using the default css which is also what I am using on my site.
The relevant styling declaration is as follows:

body {
	background-image: url("/images/grey.png");
        color: #555555;
        font: 14px/21px 'Open Sans' sans-serif;
}

On my website when looking at the css in firebug, ‘Open Sans’ is the active font while ‘sans-serif’ is greyed out and vice-versa on the other website.
I even swapped out the css on my site for the one from the other site but it made no difference. I can just delete ‘Open Sans’ or change the order so that’s not the problem, I’m just curious why they are not using the same fonts.