I have an idea whats going on.
The billion is syncing higher so you get the higher throughput, you wont get 71mbit throughput on a 71mbit sync speed.
Why is the asus syncing lower, different chipset maybe, but also it looks like you have enabled some FEC overheads in the asus DSL setting overiding the dslam.
If you want to use the ac68 as your router, then setup the billion in bridge mode as the modem and use the asus as a router only, not also as the modem.