It's probably much like gunshot: the hit may mess you up inside real bad even if the puncture wound is insignificant.
The Jem'Hedar carbines seem to work that way. Redshirts tend to die quick, but when we are supposed to care about the character, they go more slowly. That one redshirt in "The Ship," I think it was, died after several days of internal bleeding. Eddington didn't go down until after several shots, and Sisko acted like he could have survived if he didn't stay to be the hero. Nog had to get his leg amputated.