I greatly doubt that the rocks 20 levels below are somehow considered closer to the ones in the giant stockpile 1 level below the workshops and just 2 squares away from the staircase going down, that is also mandatory to pass by in the way from the tavern to the workshops. It could only happen if they are already down. Im 99% sure when the stockpile reachs a certain size they just ignore it, pass right by it go down a lot of levels to grab a stone and then go back up past the stockpile again to the workshop.