i have to say that this preview thing is bullshit. For example, your web-based iPad viewing size is only 582px wide, when in all reality, the iPad is 1536px wide. Sorry, but this little test is an epic fail.
The aspect ratio of the browser's viewport and of the device's display IS NOT the same. (That applies to portrait and landscape too.) The interface elements at the top and bottom skew that quite a bit.