Mythologies

The items, like the Heroes, comes from a certain mythology. There are 6 and we can know it by observing the background of the image or its shield:

When you equip an item to a hero of its same mythology, it gains an additional percentage bonus to the stats the item boosts.

Example: a whip from Egyptian mythology normally increases damage by 200 and strength by 9%, but if you equip an Egyptian with it, you get an additional 10% bonus to the percentage itself, rounded up in this case would be: 220 damage and 10% strength