How come Sion W passive doesn't work like Garen's W?

Sion's W passive gives him max health for every unit he kills(and for assists vs champions). Garen's W passive grants him armor and magic resist for every unit he kills(and I think for assists vs champions). Garen and Sion's W passive works like this: if they have no points in it they won't gain anything HOWEVER for Garen once he puts a point in his W he will gain all the armor magic resist from units he had killed before any points put in it. Say Garen has killed 20 units and his W grants him 0.33 armor and magic resist per unit kill. Once he puts a point into his W he will immediately gain 20x0.33 armor and magic resist for the units he killed before a point was in his W. For Sion it doesn't work like this. He has to have a point in his W to start getting credit for the max health increases. My question is why do they stack differently for those 2 passives?
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