Linux on Hikey960 has no support for USB 3.0 and many hardware features of Hikey doesnt work.
The RPB snapshots use a close to mainline kernel (v4.13 plus in-flight patches), effectively showcasing the current progress upstreaming support for the board.
AOSP master uses a different kernel (v4.9) together with drivers that may not be upstreamed in their current form (and, indeed, device tree bindings that may not be upstreamed in their current form).
Thus we expect there to be a different levels of feature enablement.
Note that RPB is a source release; you are welcome to modify the recipies to use the Android kernel instead as a basis for RPB.