[css2.1 paged media] percent page margins

Unlike regular box model, percentage values of page margins are not from width but from the direction of margin (width for left/right and height for top/bottom).

I realize this is too late to change, there must be implementations with this definition (are there?)... but am just curious why this has to be inconsistent with box model elsewhere? I am not sure I see use cases that are strong enough to make it different...

Or is it perhaps designed to produce same computed margins on portrait and landscape pages?