[Edi]

Note that bile is a poison breath attack, so it does not do direct damage but is applied over time instead. This gives the dragon's targets time to attack it in melee and possibly do significant damage to it.

The frost breath and fire breath of the blue and red dragons do direct damage and thus eliminate any units they hit instantly if the damage is high enough.

As far as the Dominions strength difference, that may or may not be a mod error. I don't know.