It appears that aircraft receive an undisclosed penalty to their ground combat scores based on how many attacks they have. As shown in the attached image the listed soft attack/defense for this particular tactical bomber is 1364 per attack with 2 attacks per combat round. The management window lists this as 2728 soft attack/defense as is expected with the new update in 1.08.06 that premultiplies the combat scores based on the number of attacks.
However, this is not the issue. The issue is that this premultiplied value is lower than it should be, as revealed in the design log. According to the design log the soft attack of this model, after all other relevant and known bonuses and penalties are applied, is 3637. When we go to apply the multiple attack (2) divider to this the final per-attack soft attack should be 1819, not 1364. The actual value is 25% less than the expected value. There is an increasing penalty that aircraft, and only aircraft, experience as their number of attacks increases: for 2 attacks it is a 25% penalty, for 3 attacks it is 33%, 4 attacks is roughly 48%, 6 attacks is roughly 55% to 58%. These penalties are consistent across airframe size, mission role, power-to-weight ratios, etc.
Was this an intentional addition when aircraft were developed and added to the game? As stated only aircraft have this penalty; artillery, rocket artillery, missile launchers, walkers, ICBM's, any ground model that has multiple attacks does not have this penalty.