The cheapest poorest performing composite boards tend to have thin walled products that have a rectangular hollow. These will split and fail over time. To overcome this a compromise between wall strength and expansion needs to be achieved.
The best design on the market are boards with round hollows in them. Like an arch of a bridge, the round holes allow for the even distribution of loads, creating a stronger board, like stronger bridges.
The holes will also allow for the reduction in expansion and contraction, taking any expansion within its own voids. This means a strong board is produced that will not expand.