Is damage taken consistant?

How come Tahm Kench's grey health that is supposed to grow larger off of damage taken is made smaller by other sources of healing than his e and isnt made larger by damage shield by taking damage while other shields shield him e.g.(janna's shield, locket of the iron solari's shield) but zeds damage on his ult still accumulates through shields even though it scales off of the same damage taken by the enemy champion .