Safari on an iPhone will display the first 320 horizontal pixels of a background image

I made this image (a .gif of 750x1000 dimensions) and used it as a background at a web site.

 
 

I noticed that when I held my iPhone vertically that I seemed to see the first 320 horizontal pixels of a background image enlarged twice over covering 620 pixels of horizontal space. I emailed myself a screen grab. It is a .png of 640x960 dimensions.

 
 

When I resized the image in Photoshop to 320x480 (half the size), I get an active area of roughly 320x356 pixels which corresponds to the upper left 320x356 pixels of my background image. I laid the image below over the original background in Photoshop, positioning it at the upper left with the thin black line at the top and the menu stuff above it off the top of the layer. The image matched up to what was beneath it on the original background.

 
 

Turning my iPhone horizontally, I get what turned into a 960x640 pixels-in-dimension .png when I sent myself a screen grab.

 
 

When I reduced the image to a 320x213 size (one third original size), I got an active area of roughly 320x139 pixels which again corresponded to the upper left of my original background.