Why? https://www.google.com/search?q=receiver+mixes+up+channels&ie=utf-8&oe=utf-8 shows it's not specific to your model but rather sources that look to be mixed up.
If I take a bog standard BluRay 5.1 source over HDMI (not downloaded but from disc) I've yet to hear this issue. When you toss a PC in the mix, it's been the PC at fault. Drivers most likely.
i am from Austria and i bought this nice surround receiver "LG-AR702 NS". I bought it used last year. I'm very pleased with the sound and i really like it.
Here comes my problem. LG Austria, Germany .. do not support this product anymore. You can't even anything about it at LG-Online in my country. Its' like it didn't ever exist.
So my problem is, that when i connect my PC or PS3 directly via HDMI to the LG, there is this problem, that it recognizes the "multi-channel" input, but messes it up, and center comes from surround right. I got this problem on PC when using windows-sound-configuration. when i click on the center, the test-sound comes from the back. When i watch a downloaded 5.1 video (movie-trailer), the sound goes right. But games, programs and windows do not work well.
Same at PS3. Netflix works fine. Amazon Prime Video not. Netflix even works fine, when i got 1. PS3 to TV via HDMI. 2. TV to LG via Toslink)
I think it depends on the signal that comes into the AVR. I found out, that there was a similar problem with "Denon" receivers. This could be fixed with a firmeware update. But i couldn't find anything for the LG.
Do you have any idea in fixing this?