Hey Crimson,
They are based on the growth levels and the difficulty on challenges with more than 5 zones may appear insane, but that is their intended difficulty level - and you aren't the first person to think it was a bug because of the steep growth.
However, I have some plans coming after this Easter event that will increase damage a good amount and give a stronger 'goal' based gameplay mechanic to the game. I'll be discussing it fairly soon. There's additional changes coming to Challenge Milestones, and level 100+ damage bonuses on top of it all.
They are based on the growth levels and the difficulty on challenges with more than 5 zones may appear insane, but that is their intended difficulty level - and you aren't the first person to think it was a bug because of the steep growth.
However, I have some plans coming after this Easter event that will increase damage a good amount and give a stronger 'goal' based gameplay mechanic to the game. I'll be discussing it fairly soon. There's additional changes coming to Challenge Milestones, and level 100+ damage bonuses on top of it all.