The nuts are discrete, point-like objects. But length is continuous, and so is area. The whole point of the artwork is to point out that its possible to confuse discrete objects with continuous objects, and confuse 1 dimensional objects like lengths with 2-dimentional objects like area.....and have off-by-1 errors, and double-counting errors...basically, you can make every mistake that it is possible to make.....
....AND STILL not realize you are wrong, because in this particular test case, the numbers came out to be what you expected them to be. Its saying don't do that.