It does seem strange. But if there's anything I've learn about watching anime, it's learning to role with the weirdness. I don't go to them for any sense or logic; as long as it follows it's own story consistency. Personally, while I do like the idea of giants terrorizing humanity and humans ability to fight back, I think they're going at an oddly inefficient way about it. I mean, slicing the back of their necks? It's badass, but you'd think you'd want to invest in stronger ranged weapons for that sort of thing. They've already build cannons and have working muskets, they just need a higher caliber. Granted, I know why they need to cut the nape of their neck, but I still don't see why you can't get the same results with a well placed cannon ball either. Though I do admit, the characters themselves... Not something I hold dear to. They have their charms, but honestly, I wouldn't shed a tear if they get eaten.