"When you Preview your content, GWD adds a lightbox CSS style that hides any overflow so you get a sense of what your content will look like within a specific viewport size. It masks/clips like you expect.
However, when you publish, GWD does not export this style since an ad may be authored for multiple device sizes utilizing content-aware techniques to show more content for bigger devices. Since most ads will be shown within an iframe or web view of a set dimension that you specify (like in DoubleClick Studio), the overflowing content will be hidden by that view."