the css property of <hr> are :
hr {
-moz-border-bottom-colors: none;
-moz-border-image: none;
-moz-border-left-colors: none;
-moz-border-right-colors: none;
-moz-border-top-colors: none;
border-color: #EEEEEE -moz-use-text-color #FFFFFF;
border-style: solid none;
border-width: 1px 0;
margin: 18px 0;
}
It correspond to a 1px horizontal line with a very light grey and vertical margin of 18px.
and because <hr> is inside a <div> without class the width depends on the content of the <div>
if you would like the <hr> to be full width, replace <div> with <div class="row"><div class="span12"> (with according closing tags).
If you expect something different, describe what you expect by adding a comment.