If it is a piece of proofgrade, you won’t be able to go to the left and right, bottom and top margins of the wood without shifting it.
Is this what you are talking about?
Short answer is that bed size and tray and machine size were hardware choices that gave a 20x12 physical bed but in product development, the software and system requirements meant that actual laserable surface area is smaller. It gradually increased over the rollout, but at the moment is pretty stable.