Is iPhone's Safari resolution treated differently than the rest of the device?

I have an image that is 480x270. In theory this should occupy all the space between the battery bar on top and the navigation bar at the bottom when the phone is rotated on to its side. However, inexplicably the image is displayed shrunken to where it barely occupies 1/2 the screen horizontally.

I thought perhaps the phone is shrinking the image fo开发者_如何转开发r some reason, so I just created a div that has its dimensions set to the same size as the image and nothing in it but a background color. The phone displays this rectangle of color in exactly the same dimensions as the graphic.

So, the question is, is Safari compensating in some way and displaying things in a reduced ratio, and if so, WHAT is that ratio?! (or am I just totally out of my mind?)

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Check out viewport.