This isn't a bug; I encountered the same issue. I spent so much time on the hot seat that one of the heroes ended up with 200+ champions, while the other had 400+. I look at the map... "High," "Deadly," and in some places "Medium" danger levels. Hmm. I managed to improve the Dynasty weapons on "Medium," but I was surprised by this outcome.

Then I realized why this happened: a hero's army grows by +n per week, while neutral forces grow by +n% per week. In the Archangel scenario, I calculated the percentage to be 17. Therefore, the neutral army increases much faster each week than the hero's army.