Dolby digital comes in a number of formats:
1.0
1.1
2.0
2.1
4.0
4.1...........so on

If you look on the box it may tell you that it comes in Dobly 2.0 and due to the setting on your AVR it converts it so that the dialog comes from the center channel and other sounds will come from the left and right.

The Dolby Digital just means that the sound was done in a digital format not full 5.1 or higher.