#masthead{
position:absolute;
top:0;
left:252px;
}

No width specified therefore object width conforms to its contents.

#left{
position:absolute;
width:252px;
}

Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat. Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi.Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat. Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi.Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolor

#center{
position:absolute;
top:120px;
left:252px;
width:100%;
}

100% width specified. Hence this object uses its parent object - the browser window - to base the measurement against. As you can see, this object exceeds your windows width by the 252 pixels specified by the the left column.

#centerBot{
position:absolute;
top:300px;
left:252px;
width:auto;
}

Auto width specified therefore object width conforms to its contents.

#bottom{
position:absolute;
top:470px;
left:252px;
width:65%;
}

Width specified at 65% as determined by remaining window space after subtracting the left columns fixed width from the entire browser width. Notice how the right "margin" is inconsistent as you size the window from left to right.