Sorry, but I'm stumped. I'd suggest you head over to AVSForums.com. Their members deal with this stuff on a daily basis, and could probably explain the reasoning behind this.
Being a big fan of audio (2 channel and HT) I do know about AVS. I just seem to have switched my interests these days towards displaying my media everywhere.
I have found recently that Panasonic TV's are a bit picky about display settings. My guess is that it's a setting on the TV, not the computer. I don't know TV's as well as I know computers, but you should try checking the advanced input menu.