How would adding allies, and thus more units to your battle roster, decrease your A/D score? Here's a hypothetical example.
1. EXISTING: You have 10 allies so you can bring 40 of your own units into battle (4 units per allies). Let's say you have an A/D score of 1,000/2,000 and 44 units. Four of the lowest rated are Light Gunners, with an A/D score of 1/0 so right now they aren't part of your battle roster, because you can only use 40 of your 44 troops.
2. HYPTHETICAL: You add one allies, so you would get to bring four more of your own units into a battle, those four Light Gunners now join the battle. So your A/D score would go from 1.000/2,000 to 1,004/2,000.
Why would the A/D number go down if you add more allies and thus more units?