After looking at the immense amount of guessing from last year and going back through the recent comments, here, I must present my own hypothesis.
It is thus (and please pardon me if this has been mentioned already before):
The wing isn't so much "flexing" as it is "twisting".
As mentioned before, having the wing pitch downwards under high-speed loads would be an immense challenge bar the "inserts" seen on the Mclaren and FI(?).
The new FIA tests are to apply a load, I'm guessing, to provide a pure bending moment to the wing. What I imagine is that the materials engineers at Red Bull have laid up the carbon fiber to provide maximum resistance to those loads, but perhaps provide less resistance to twisting (torsional) loads. This would allow flexing at high speeds, with the wings twisting, but can still withstand FIA loads.
If I haven't over-explained myself enough, it would be like the front-wing elements pivoting about a solid bar.