This has nothing to do with the zoom, but with the current width of the window, ( At least in your examples, the same happens when you change your browser's window to a smaller size ) they are most likely using CSS media queries to apply conditional styles depending on the width.
Sample ( from first google result http://css-tricks.com/css-media-queries/ )
@media all and (max-width: 699px) and (min-width: 520px), (min-width: 1151px) {
body {
background: #ccc;
}
}