They have no DT to begin with. Are you saying that reducing it's health to 10 (from 15) would make a difference?
I don't really know the reason, but I made some custom hats, a bandana, and glasses (nothing special about them, just added a custom object effect), all have 10 health, and as far as I can tell, they've never been damaged.
just try it, make yourself a new hat and go get shot in the head (you're more likely to get hit in the head when crouching) :laugh: .