All object hardnesses are nothing more than arena settings. You can set them to whatever you want in a custom arena, and we CM's sometimes change them for Special Events. It would require a little work for HTC to set each Main Arena differently, and it's very possible that when the arena resets after a map is won that all settings revert to default. I'm guessing the default is the same everywhere and someone would have to manually change them every time to something different.