I hate phalanxes

[Cadwalader]

I have recently started playing EB again after a long absence, and noticed that in my Bactrian campaign, my severely weakened unit of levy phalangites absolutely crushed a full strength Seleucid unit of pezhetairoi! While this event probably gave me the victory, it seems highly unnatural even though my unit was defending. I've made a few experiments in custom games, and it seems that the phalanx tries to actually kill people only some of the time, and it appears to be completely arbitrary whether they choose to do so or not.

..Or is it? I was hoping someone could answer that.