Xin Zhao [W] Incorrect Healing

Throughout my experience playing Xin Zhao , I have come to notice that his [W] heals less than the amount indicated by the tooltip. I have tested multiple times and came to the conclusion that it could be a bug.

In the attached image, the tooltip for Xin's [W] indicates that it will heal for 90 HP. However, the image captured showed that only 98 HP was gained with the 20% bonus from Spirit Visage and 8% from Runic Armor mastery. That amount is far from what it should have been if the tooltip is to be accurate. Without Spirit Visage, Xin's [W] will heal A LOT (ok maybe just significantly) less than indicated by the tooltip.

Please verify whether this is an actual bug or if there is certain mechanics of which I'm not aware.