If you pause it at 0:03 you can see the blood splatter, it seems to indicate it registered that you shot the player in the shoulder(yes I know they moved slightly after the shot). Which could be from inaccuracy of the rifle (it's not ever 100% accurate, there can be slight differences, and the farther you are away the more those small differences can change where you hit).
Both of them likely registered as upper arm shots do to the weapons they were holding, and minor inaccuracy of your rifle. After all, the arms were reaching out towards you, so that makes the chances of you hitting an arm hitbox higher.
Here you see the default hitboxes:
Closer look:
As you can see the arms slightly block parts of the head, the upper body hitbox also extends up into the head a bit, meaning depending on the angles you could register a body shot on headshot.
Also do note, any hitbox OUTSIDE of that yellow box will NOT be shootable. Meaning you will 100% shitbox on any limbs sticking out of that. The bullet must be inside that yellow box to even check if it hit a hitbox. This is for optimization purposes. It's much easier to check for a raytrace against a static box, than rotating boxes. (The players bounding box (the yellow box) does not rotate).
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