But it's wrong.
My computer doesn't weigh 7 shoes just because I say it does.
It was just a statement. The game still operates on real world physics. Three feathers do not weigh the same as 1 mammoth tusk.
The game operates on real world physics, yes; it just doesn't mean real-world measurements have to come into play. It's like how they always get around calling English "English" in works of fantasy, because in that universe there's no England. It's always "Common speech", or "Basic speech", or something to that effect.
Just as there's no England in most works of fantasy, there is no metric system.
My point is that you can call it whatever you want, and it doesn't necessarily have to be compatible with the real-world object.