What if we assume that 4 hate = 9 healing aggro? (1 damage aggro ~ 2.25 healing aggro). It would actually be closer to 4.1 and the gradation would be slightly less. Yes, it's all guesswork =) And no, I don't like this number either, I feel that Blizzard likes to use more round numbers.
The only thing I can think of is that Healing spells, much like Warrior damage abilities, have some innate threat built in, and a bit of extra threat from healing itself. So for example, "original" 1000 healing aggro listed in that threat is actually 500 natural aggro and 500 aggro from the 1000 point healed. This would explain how Paladins are able to have lower hate heals as compared to other classes - their innate hate for the heal can be simply 0, so paladin's 1000 point heal will only cause 500 aggro. Similarly, supposedly renew has about half the hate of regular healing spells, which would be explained by this concept. Isn't making up theories wonderful?
The only thing I can think of is that Healing spells, much like Warrior damage abilities, have some innate threat built in, and a bit of extra threat from healing itself. So for example, "original" 1000 healing aggro listed in that threat is actually 500 natural aggro and 500 aggro from the 1000 point healed. This would explain how Paladins are able to have lower hate heals as compared to other classes - their innate hate for the heal can be simply 0, so paladin's 1000 point heal will only cause 500 aggro. Similarly, supposedly renew has about half the hate of regular healing spells, which would be explained by this concept. Isn't making up theories wonderful?