Healing (Hero Ability)
It resurrects 2% + 1% per level; However it always rounds down the number of units resurrected instead of rounding up.
Example: 93 Nymphs are slain during battle. With level 4 healing, 5 Nymphs would resurrected from death. Actual number is 5.58, but instead of rounding up, it rounds down.
So it only resurrects a full whole number of units. I.e., at lvl 1 (3%) of Healing, you need to lose 34 of a unit to gain one back. Which is why it's not often seen resurrecting ultimates.
Heroes with ability