iPhone video specifications

I downloaded a TV show from iTunes that has a resolution of 853x480.
According to the iPhone tech specs, 640x480 is the maximum resolution allowed for iPhone video.
However, iTunes allows me to sync it with my iPhone.

I have my own personal video that is 720x480 (in Quicktime format).
iTunes will NOT let me sync it with my iPhone. To make it iPhone friendly, iTunes lowers the quality to 640x426.

How come iTunes has no problem syncing the 853x480 video? It clearly breaks the limit set in the Tech Specs.