Why does Grasp of the undying scale with Max HP?

This is a mastery for tanks so why not let it scale with bonus hp instead. As it it right now even non tanks can just take it for a nice bit of early game damage and sustain.

At lvl 1 has 616 hp so grasp deals 18 damage(18.48 to be exact) At lvl 1 has 558 hp so grasp does 17 damage (16.74 to be exact)

is supposed to be tanky, isn't yet the difference is so small. Now u might say well what about hp gain per level?

At lvl 3 has 741 hp so grasp deals 22 damage(22.23 to be exact) At lvl 3 has 685 hp so grasp does 21 damage (20.55 to be exact) Still not much of a difference

At lvl 6 has 949 hp so grasp deals 28 damage(28.47 to be exact) At lvl 6 has 898 hp so grasp does 27 damage (26.94 to be exact)

If it scaled with Bonus hp then the mastery wouldn't have had to be nerfed because everyone and their mother was using it in toplane. It would actually be good if on tanks while being crap on squishies which is how is was meant to be.

Edit: For heal values just halve the damage values.