I explained issue in one other thread.

The problem is that spell reduces base attack of unit, while on the other hand, when penalty is calculated it takes total attack of unit in consideration.

So unit with lots of strength has big total attack, but not so great base attack. When base attack attack is smaller then half of total attack (which is pretty much with every unit with strenght over 20) you'll get negative value.

Similar issue exist, if confusion is used against unit groups, since it's also applied per unit base attack, while calculated using total attack of the group.