The offset for dialnorm has been changed from -31 dB representing unity to -27 dB representing unity in order to put Dolby Digital at the same average playback level as DTS. This means if dialnorm is -31, there will be a +4 dB boost. This has carried over so even Dolby TrueHD, Dolby Digital Plus, and DTS-HD content (DTS-HD HR and MA can contain the dialnorm parameter) are offset.
I don't know if that change was originally mandated by Dolby, THX, or something manufacturers have done themselves. My Yamaha receivers have both done this.
When it comes to dialnorm, it's important to remember that it doesn't only indicate if the soundtrack should be attenuated, but it also marks the center of the null band for dynamic range compression upon playback, protecting that region. If it's set arbitarily, the DRC that's part of the Dolby codecs won't perform well for those that need or want to use it. The behavior of DRC (amount of compression and the size of the null band) is controlled by the DRC profile applied during encoding. DRC can be defeated in encoding if desired, but the manufacturers' own night modes may still use dialnorm for reference.