another thing as well I've noticed with my own boards....
Having more copper on one layer, tends to make my board warped. So maybe in this situation, they tried to balance the amount of copper on each side. But, I have really only noticed this happening with my boards when I do a solid layer on one side, none on the other.
It's like having a sheet of 1/4" plywood. Then gluing Laminate to one side. It warps like crazy. But when you glue Laminate on both sides, it stays flat. The surface tension of the glue/laminate pulls in both directions when laminate on both sides, making the piece stay flat.