This felt like the most obvious explanation to me as well. Maybe the artist's vision for it was a solid cube of cash, but it ended up needing a structure inside to support the thing.
So many reasons this might be exactly $1,000,000 but not sum up on the outside.
That said, this is also something I would have spent way too much time overthinking, so I thoroughly enjoyed reading the blog post.
Artist should have been fired. If I'm being shown a stack of $1 million, it better be a stack of 1 million of they are gonna be talking with the fishes.
