Scattering.

There is often situation where the chance to hit is 1%, and chance to scatter is 10%, or even more. And if there several units, surrounding the target, than chance to hit anybody increases really high, even if you try to shoot thru whole map. =)

I attach an example. Not the best one (because I play necro, and human archers tend to have higher chances to hit in same cases), but you get the point anyway.

By the way, that is the situation right after deployment, so it is a shot thru whole map.

[Legulysse]

This is the intended mechanic.
It allows you to increase your chances to hit "somebody" on a difficult shot. The more the enemy is packed, the more units are added in the equation, so the more chances to hit somebody. (not sure about my English here !)
The chances to hit your target can go down to 1%, but the scattering will provide an additional ~10% for each unit, divided between the potential victims depending on stats.