5.1 Source playing as 7.1

[pat bateman]

I've never noticed this before. Denon X3700, and maybe this is a dumb question. But why if the input source is 5.1, and no up-mixing option is selected on the Denon, is it outputting 7.1?

 

[Thonolan]

I think this has been discussed before multiple times. For instance, my Marantz Cinema 40 always outputs 5.1 mixes in DTS or DTS-HD Master Audio as 7.1. However, it outputs them strictly as 5.1 when it's LPCM or a multichannel SACD source.