Why that is the case only Onkyo and Dolby can say. Like I mentioned earlier, IMAX DTS did the same regarding channel behavior. Perhaps Onkyo thought everybody at some point would use an Apple TV 4K for streaming and never watch broadcast TV or play discs ever again with dolby 5.1 signals. “What does that have to do with anything?” you ask. Here we go.\n\nThe Apple TV 4K converts everything to LPCM by default but retains Atmos metadata(Dolby MAT) for processing by a receiver when getting an Atmos\/DD+ 7.1 track from a stream. Anything in 5.1 gets sent as LPCM and the receiver simply recognizes the signal as MULTICHANNEL PCM. Since there is no Dolby signal flagged in the receiver, an Onkyo receiver specifically, it will output surround channel information into the surround speakers in DIRECT or straight MULTICHANNEL PCM sound modes with a 7.1 configuration.\n\nOne could work around the issue with PCM conversions in disc players for 5.1 but TVs and streamers will convert down to two channel PCM. They could also change the configuration from 7.1 to 5.1 for those dolby signals. Hey, they have to go into the Setup to turn on Center Spread anyway.;) It is what it is and is an understandable deal breaker. Just do the homework on these things before making any deals. The good stuff is in bold print. The bad stuff is hidden away here and there.