The "normal dialog" level was chosen to be -31dB. If the average level of the dialog on a dolby digital track is not -31dB, then there should be a DialNorm value in the bitstream that specifies what the average level is. The decoder will then boost or attenuate the average level so that it is back to -31dB.
For example, if the DialNorm value says that the average level of the dialog is -27dB, the decoder will attenuate the level by 4dB to get it back to -31dB. This is Dolby's attempt at dealing with wildly varying levels on digital tracks by automatically making the levels of different tracks similar (so you don't get blasted when going from one DVD to another). It relies on recording engineers properly specifying an ACCURATE value for DialNorm and they don't always do that so the result varies.
