How come air flows in under the fuel shelf?

Hi

According to this image, air intake is supposed to flow from below the fuel shelf:



Why is this the case? Air would also flow in from above the shelf - right? In fact, wouldn't most of the air flow in from above since that's where the flame is (shortest distance)? Air would need to take a longer route if it comes in from below the shelf.

Thanks

Air comes above and below the shelf. But above the firewood blocks the tunnel partly, and the air takes the path of least resistance, under the shelf.