OK, so I watched some more because the seat belt thing bugged me. And I think I found something.
When the 16 and 17 side curtain airbags go off, they end up getting between the dummy and the belt. This may be forcing the belt down the torso of the dummy, and reducing the belt's effectiveness.
Here is a view of the belt during impact:
And from the outside:
The 17 also appears to have a delayed activation of the side curtain relative to the steering wheel air bag, this was not present in the 16 test, but on the 16 the belt also ended up on the outside of the side curtain air bag:
Now contrast that with the IIHS 16 side impact test:
Here is the side curtain inflating on the correct side of the seat belt.
From the rear seat view, you can see how the bag tucks in outside the seat belt:
What is the difference (other than crash direction)? The side impact has the windows closed, the small offset has them open.
So my questions are: Are the side curtains designed to operate best with the windows closed? Is the testing methodology impacting the results? And if so, how much?
Or is the issue the belt is not tightening (pre-tensioning) enough to counteract the side curtain airbag?
And in case you're wondering, the moderate offset test is windows down and also catches the belt.