As I understand it, our processors at home (AVR, Blu-Ray player, etc) automatically lower the gain based on the difference between -31 and whatever the assigned dialnorm value is. So if it's set to -31, the overall volume is reduced by 0 dB. But if it's set to -18, the gain will be reduced 13 dB.
I first discovered this when converting my own Atmos mixes from ADM to TrueHD with the Dolby Media Encoder and then playing back the resultant .MLP files with the Reference Player. Once the TrueHD file is created, it appears that the assigned dialnorm value is permanent.