hand foot major wound knockdown stunning

[Captain Joy]

The italic emphasis below is mine.

1. Injury over IP/3 cripples a hand or foot [B420-1].
2. An injury that cripples a body part also counts as a major wound [B420].
3. Whenever you suffer a major wound you must make an immediate HT roll to avoid knockdown and stunning [B420].
   • Success: ordinary shock penalties
   • Failure: Stunned, drop anything you're holding, and fall down
   • Failury by 5+ or critical: You fall unconscious
4. After combat you make a HT roll to see how serious the wound is [B422].
   • Success: Full recovery after normal healing
   • Failure: a lasing injury, but ultimately a full recovery
   • Critical Failure: The hand/foot is permanently crippled and you gain the appropriate disadvantage

A red shirt in my Star Trek campaign recently got a harpoon through his right hand. I remembered the first HT roll right after he received the crippling/major-wound injury. He made his HT roll. But, had he failed it, he's really supposed to suffer knockdown? From a hand injury??? Such knockdown seems a reasonable for major wounds to the body or legs, but not arms or hands. (Not sure about a foot.)

I encourage one and all to explain to me why it's realistic for a person that gets shot in the hand to fall down about half the time. Or, if I've got the rules wrong, please enlighten me.

(The player still needs to make his second HT roll to see how serious the crippling injury is.)