My guess would be that the pencils of the same property are on each shelf, and the older stuff are buried under newer ones.

For example, one of the labels says "oily pencils".

So perhaps when you go to him and ask for a purple pencil he pulls out a number of purple ones from the bunch that's relevant to you. Then he explains which one is better for you.

Kind of like Ollivader from Harry Potter haha.

Came for this.

“Curious how these things happen. The [pencil] chooses the [writer], remember…”