They’ve left a gap for the draw that is meant to represent 0, which in itself doesn’t make sense, and additionally the gap is smaller than the drawer that is meant to represent 1. There is no second drawer equal to 1.This makes it clear that drawer number 2, is not in fact twice the width of the drawer that represents 1, as it is only as wide as a single drawer representing 1 and the lesser space representing 0.

Additionally there are only 6 drawers, so even with the missing 1, they don’t reach 21, but rather 13.

Hats off to you, Sir.

The drawer’s ratios are squares at 1, 1, 2 (smallest drawer built), 3, 5, 8, 13, 21 (largest drawer built), all of which are Fibonacci series numbers. 

Here’s an image so you don’t have to look it up: http://upload.wikimedia.org/wikipedia/commons/thumb/9/93/Fibonacci_spiral_34.svg/180px-Fibonacci_spiral_34.svg.png

Phi is defined to be the unique number such that if a rectangle’s sides have a ratio phi, and you remove a square based on the smaller side, the remaining rectangle’s sides have ratio phi.  It is no less cool than the Fibonacci series, by the way; look it up.