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Device-width refers to the display's resolution (eg. the 1024 from 1024x768), while width refers to the width of the browser itself (which will be different from the resolution if the browser isn't maximized). If your resolution is large enough to get you in one break point, but the width of the browser is small enough to get you in another one, you'll end up with an odd combination of both.

Unless you have a legitimate reason to restrict the style sheets based on the resolution and not the size of the viewport, then just use min-width/max-width and avoid min-device-width/max-device-width.