The visual viewport is the part of the page that’s currently shown on-screen. The user may scroll to change the part of the page he sees, or zoom to change the size of the visual viewport.
However, the CSS layout, especially percentual widths, are calculated relative to the layout viewport, which is considerably wider than the visual viewport.
Thus the element takes the width of the layout viewport initially, and your CSS is interpreted as if the screen were significantly wider than the phone screen. This makes sure that your site’s layout behaves as it does on a desktop browser.
How wide is the layout viewport? That differs per browser. Safari iPhone uses 980px, Opera 850px, Android WebKit 800px, and IE 974px.