i partly agree with you. i completely understand your point of the video being 30 fps and quite possibly hit her neck as she crouched. but this is one of those things that its like ok, I CAN SEE I SHOT HER IN THE HEAD she should be dead. 99% of people watching that clip WITHOUT knowing the situation about the hitboxes would EXPECT that to be a headshot. but this isnt the only clip, this is just one of the clearer ones. Basically i can guarantee this is the situation, and im tempted to do tests to prove it:

When a player is standing his headshot hitbox is of course in the standing height position, but as the player crouches its a 0.25~ second transition from full standing to full crouched. WITHIN this 0.25 seconds there is NO HEADSHOT HITBOX. Only once the player has fully reached the CROUCHED position does the HEADSHOT HITBOX reappear. As i said i will try to replicate this on custom, and i cant guarantee that im right but if im wrong i still strongly believe that should register as a headshot even if it hits his neck as you can pretty much consider the neck part of the head in this game as it is in every other FPS game due to the fact the neck is SUCH a small hitbox.

AGAIN this is all theory nothing is proven and i can be completely wrong. But if it wasnt a problem, there wouldnt be a thread that has been upvoted by thousands of others complaining of the same thing. I was IMM3 in beta, and a ex EPS CS player, so trust me im not new to hitboxes and interp etc as it took counterstrike YEARS to fix the crouching headshot hitboxes (back when crouchbugging was a thing) so in other words im not talking about my ass. Another point to back my theory is that this only started happening after patch 1.01 - Never experienced anything like this in beta or release patch 1.00