Does the AI get hidden bonuses not listed on any documentation?

[Ludens]

I don't know what caused this, but I am pretty sure it's a bug and not a feature.

I vaguely recall some issues with the min. angle setting of projectile weapons that prevented units from shooting downwards. However, it would require a huge height-difference to account for your horse archers being unable to return fire. It would not explain the skirmishers' range, either. AFAIK EB hasn't changed the minimal angle settings, anyway.

[CaptainCrunch]

My horse archers were actually firing just fine, the problem was that the skirmishers could return their fire as soon as the HAs were at their max range. The height advantage of the HAs may have been ~4m to scale. So far I haven't experienced any problems with missile units firing from a height advantage, but as you explained it appears that the height differential has to be pretty significant. In my experience with the game even a slight height advantage can net huge tactical superiority with archers.

I figured it had to be a bug, but if there's ever a next time I'm getting a screenshot. Thanks for the input.

[vollorix]

I´m not sure, but it could be, since the enemy units was in a forest, that one of the men had a pathfinding problem among the trees, and therefore was much closer to your unit than the rest of his comrades.