Comment by stn8188

The sibling reply is certainly correct - all those high speed signals need room, plus adjacent reference planes for good signal integrity. The power distribution also probably needs a lot of copper for the relatively high current the board sees.

That being said, an odd number of metal layers is very unlikely. It's physically possible but due to the lamination process, PCBs are basically always built in an even number of layers. It's a bit tough to tell with the resolution of the scan, but it looks like the dielectric layers were counted as 9, giving 10 total metal layers.