Turn order in battle.

I can't understand the reason for this fact; maybe someone knows the deeper meaning.

The unit that retaliates receives a speed value that is essentially the same but with a negative sign, so faster units move later than slower units after a mutual wait.
The speed of a demoralized unit becomes "zero" (even less than that of artillery), and it will move before the retaliating zombie or peasant, even if those have an expert slow speed applied; their speed will be equal to "1".
However, if both units are demoralized, then the law of sequence will apply – whoever retaliates first will move first after the wait.