Hmmm. Is this independent of the Weapons they're using? For example, would that also be true if a Glad or Pal was using the same Weapon as the Assassin?

Train (or edit) all your characters to different classes, give them the same Weapons and stats, and train them to the same level and expertise. Some character classes have special traits, so avoid training which would bias one over the other. Then we can talk about this much better.