In fact the dinosaur statues are not plastic. They were constructed from cast concrete in sections and supported internally by brickwork. I used to love visiting crystal palace to see them as a child.
I think it probably made sense to the author because they've used all three (-1,0,1) for other examples, and would've been fine until separated out to a method reused to show the actual number.
I think they tried to be a bit too clever, basically.