That'll be down to third party stuff Asus have done, and only recently too.
So it'll have latency added on, like the NF200 chips for 16x bandwidth over 8x.
Whereas 1155? You throw an Ivy in any 1155 board and the primary lane's PCI-E 3.0, then depending how they've done the other lanes, they will either be PCI-E 2.0/3.0
Which is why I think AMD will go for a new socket, sort their chipsets out, 990FX is just a 890FX rebadge, which itself is pretty much a 790FX rebadge.