L&LD
Part of the Furniture
I've had the opposite experience with my RT-AC66U_B1 in that the uploads are matching but the downloads are showing 20% less speed when compared to my ISP speed and/or a browser based test. As said in this thread, I am chalking it up to the limitation of my router.
It may help to note the ISP, paid for speeds on your RT-AC66U_B1 (which is an upgraded 'original' RT-AC68U w/1GHz dual-cores, correct)?