I think you’re reading that stat backwards. It’s saying that only 10% of concussions come from situations where someone is knocked out, not that only 10% of people who are knocked out have concussions.Apparently less than 10% of knock outs result in a concussion.
edit, citation:
“Many people assume that you can only get a concussion if you're knocked unconscious. But while getting knocked-out can be a sign of concussion, it only happens in less than 10 percent of cases. Everyone should know what to look for in a concussion in order to protect themselves and their loved ones.”
Do You Have a Concussion? | Concussion.org
Concussions are a form of traumatic head injury caused by a blow to the head, or even a subtle, sharp jolt. Here are signs of concussion.concussion.org