I was investigating why Zhao Yun seems so much stronger in campaign compared to multiplayer and I think this is the answer.
When upgraded to the highest level, he uses a sword instead of the spear. This sword is supposed to be armour piercing in the novel but there is a mistake in the multiplayer stats and instead it reduces his own armour by 50%. See this pic which shows him with 5% armour when sword is equipped.
Now notice at level 2 he uses a spear, armour rating is 55%
edit - corrected order of photos