I see defense as armor and such
HP as your physical constitution
As for why that's the case with these ones, it should be pretty obvious
Diablo is a giant demon
Kerrigan is a zerg
Dracula is a vampire and 2/3 of his forms are huge.
Basically it's mostly their race not their gear that's at work there. I mean they no doubt have defense too because of scales or whatnot but they're clearly quite hardy on top of that.
This is kinda how it works in Fire Emblem too. The monster types generally have more hp. Big burly men have more hp.
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No problem!
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