Repel mechanics. How exactly it works?

[ano]

I'm just curious...
It was discussed recently that dragonflies are absolutely useless against an archdevil with firebrand and so they are against equipped Vanjarl (my tests say so). But they appeared to be very effective against a Empoisoner assassin equipped with Skull staff.
So... If the mindless units always receive 1 point of damage when they are repelled (manual says nothing about that, btw), then why do they always die from being repelled by Vanjarl with Frost Brand and never die from Empoisoner with Skull Staff?
Also... How can morale 50 mindless beings be repelled at all if manual clearly states that repel is based on the attacker's morale so bigger morale means lesser chance of being repelled. I understand that the manual is just wrong.. But does anyone know how exactly it works?