When you open a HDMI audio device on the RPi the kernel automatically chooses a channel map based on the number of channels (eg 8 in this case) and the speaker allocation from the EDID.
The hdmi-codec driver has an ordered list, sorted by preference, and will choose the first entry that's supported by HDMI for the given number of channels and is supported by the AVR as well (all channels of that entry must also be present in the EDID speaker allocation block).
In this table the 6.1 entry with RC comes before the 7.1 entry with the (deprecated) RLC/RRC entries, so if your AVR announces support for both RC and RLC/RRC you'll get the former (6.1) channel map
/* 6.1 */
{ .ca_id = 0x0f, .n_ch = 8,
.mask = FL | FR | LFE | FC | RL | RR | RC },
/* surround71 */
{ .ca_id = 0x13, .n_ch = 8,
.mask = FL | FR | LFE | FC | RL | RR | RLC | RRC },
It's then up to kodi to decide how to best map the channels of the media file to those channels.
In general it's best to use passthrough for compressed audio formats as this avoids all the channel mapping shenanigans but that may not be an option on RPi2/3 as they don't support TrueHD or DTS-HD because of hardware limitations/bugs (they also can't output 8ch 192kHz PCM).
so long,
Hias