Disclaimer: I am the Product Manager for one of the encoder companies listed on the sheet you mention.
Sorry I didn’t see your post earlier, not sure if you have already implemented a solution.
I am very familiar with the AT&T PEG Uverse requirements, and would be happy to answer any questions. I can explain why 480×480 may seem wrong, but is correct if the encoder knows how to support it.
You can reach me at scott(dot)thorley(at)inlethd(dot)com.
Thanks,
Scott Thorley