Because a timber pile is actually 2x2 tiles big, so when you place it centered in a tile it will overlap 0.5 tiles in all directions around that tile. Meaning 5 tiles between the tiles you build them on will actually only be 4 tiles worth of clearance.
Worth noting that you can actually put fields between board piles without punching mould into them, which actually provides a convenienly sized spacer and a practical use for the dead zone.