There is a bonus, but it does not show up in the A/D/M stats. As described, the bonus should be one-sixth of the attack and defense factors initially, going up to one-fourth after Military Academy. The question that was discussed when it first came out was whether that was 1/6 of the currently attacking/defending unit in the army, or 1/6 of the total of all the attack factors in the army (in effect, making it 1/2 of the average attack factor). Someone ran some tests, and it seemed to be the latter.
For example, in a cav army of 3 cavalry (attack 6, easily divisible), does the one-sixth mean a plus 1 bonus (1/6*6) or a plus 3 bonus (1/6*[3*6]). Based on the tests, it seemed to be the plus 3. Effectively, it is equal to 1/2 the average attack factor for a 3-unit army. When you move into a 4-unit army, assuming you also have military academy, it is a bonus of (1/4*[4*6])=6, or equal to the average attack factor. Very powerful. However, other than the one set of tests, I don't remember reading a definitive statement of which way it is calculated, and since it is not visible, it will probably lead to endless speculation!