Now that I look at them again, I think it's intentional. The thicker board is over the spare wheel and is the one thats bowed. I'm guessing that it's thicker because it's not supported in the middle where the tank board probably is by the tank. In fact there is a plate fixed along the edge of the fuel tank board to support the spare wheel board. The plate is NOT flat and is actually recessed for something wider. Also the spare wheel board has a strip of felt along the same joint (that sits on the supporting plate.. The shape of the plate suggests a different tickness unless that's supposed to account for the felt strip.
I'm sure that neither are DIY copies BTW, the detail and fixings are too good.
Many thanks
Rich