Actually that's the case with all sorts of body armor nowadays. A bullet doesn't have to piece your bulletproof vest to kill you. The dissipated energy of the impact can mess up your organs and blunt trauma is a factor in evalutating real life armor. Blunt trauma doesn't just apply to the head. So what you said should apply to nearly all SR armor despite the fact that it clearly doesn't from my examples of SR armor basically being armored spandex.
Current and probably SR armor technology aims for distributing the force over the biggest possible surface, by hardening the materials in the millisecond the force hits and liquifying it afterwards. If the force distribution over the body is perfect, the only other thing that can add protection is padding.
Now for the head, its clothe-wise disconnection from the rest of the body and small surface limits the possible distribution of force considerably, which can only be compensated by... padding, i.e. if the head has one eighth of the size of the rest of the body it needs 8 times as much padding to only get the same protection as the rest of the body.
Now if the helmet would be connected to the rest of the body, through an otherwise flexible connection, that would get instantly rigid on impact and is strong enough to distribute the whole force to the rest of the body, now that would be a helmet. But i am sure not even SR is that far yet, also it wouldn't work with a loose skull cap
Plus, capsize matters. A helmet which protects only half a head, would only get half the rating. And since the PPP helmet has almost the same rating as a riot helmet, which is actually reinforced by worn body armor.. there quite something mystical about it...