shoddy work of squareenix

[Rakiria]

Im not understand why so big difference in quality between some hairs

Okay so this is how textures for 3D things work:

Everythign has to be laid out and painted there. So you only have so much space to do the texture on. Now, if you have a simple hairstyle, like the (I think Miqo exclusive?) one that is just straight, shoulder-lenght hair and bangs, you don't have a lot of faces to put down on the texture. It's all just one flat thing, so you have fewer pieces to texture and can make them larger on the texture itself.
Now, if you have a more complex hairstyle, you'll have more faces to put down, and each of them has fewer space there. Thus, while the overall texture quality remains the same, each individual part has less pixels on it.