Trading: Throughout the day, your shop will be visited by heroes, who will buy/sell some items from/to you. Heroes who want to buy from you remain in your shop until you sell them an item or dismiss them. Heroes who sell you an item stay until you buy it or dismiss them. See Energy for more info on haggling with heroes.
Power: The tier of a hero determines their raw power (ie without equipment), maximum level, and the power bonus that they get from wearing equipment. According to the developers, that power bonus is about 10% per tier. Hero power determines the likelihood of a quest to succeed.
Questing: Players can craft and assign equipment to their heroes, and send them on to collect rare resources, gems (in story quests), and experience. Heroes wearing equipment nearer their level are less likely to break it. After questing, heroes need to rest for half of the buffed duration of the quest, unless they have the Energetic skill. Heroes injured following a quest need to rest for twice the base duration of the quest.
I thought a bit about your picture but I never really tried to reply. I found the answer pretty easy :/
Let's start off by your first picture:
About Edouard: The set power give +25% POWER.
Power looks to correspond to a +25% buff of your item
Total power from item: 3419
Base power of Edouard: 7500
Item + base power of Edouard: 10919
Based power + itemx1.25= 11773.75
Since there is no decimal: 11774 power
So set bonus (power)= 25% of your item stat
This is why your Edouard doesn't get more stat
If I could I would only say that Strength only increase base power of any heroes but let's go some math...
Let's use theor as exemple: He has cleave (+25% strength) and a berserker axer (passif= +25% strength)
Théor lvl 25 power: 1281
25% of 1281: 256.2
We delete 25% from it: 1024.8 based power (we delete cleave power)
Let's equip him a Berserker axe: 1281+2150+25% strength
We delete Total power - 2150 - 1281= 257 (in reality 256.2)
Well, I know it is badly made but we deduce than strength is additive and also it works with the based power of any heroes.
Mojian has the same affix than cleave or any other. It gives strength bonus. That why the buff is lower for Tier 1 heroes than Tier 3 heroes.
Well to summer up:
Set bonus: 25% of your item stat
Strength: X% of your based heroes stat