Mythic bonus was introduced in Update 25. Mythic bonuses stack differently than most bonus types: see quote below.
| Originally Posted by NoWorries on March 19, 2015
| At the same time [U25] we are attempting a new approach with the Mythic aspect of loot. Our new approach will have items show up with a mythic effect that will give a bonus to PRR and MRR or to Melee Power, Spell Power, and Ranged Power depending on the slot of the item. There will be a more common lower power version of the mythic effect and a rare chaser level that has a bit more power. The Mythic bonuses from each slot stack with one another. Example: You have a +2 bonus to PRR and MRR from your shield and a +1 to PRR and MRR from your necklace, you will have a total of +3 PRR and MRR.