That might explain Clark's lack of symptoms.

Another popular theory among researchers is that the stark cognitive difference between players like Clark and McNeill could boil down to genetics.

Small points out that when it comes to Alzheimer's disease, which shares some symptoms with CTE, there are certain genes, like ApoE4, that predispose people to worse cognition.

So could McNeill be genetically more vulnerable to cognitive problems? Or was it the sheer number of hits he took to the head compared with Clark?

"Everything needs to be put on the table," said Guskiewicz, a concussion expert. "It may be something environmental, it may be genetic. What we need to focus on is what creates this perfect storm."

Small is realistic about the study -- despite his confidence in the early findings -- and the need for much more data to explain what that perfect storm looks like.

"I would say to (Clark), 'Yes, I know that you have a scan that looks different,' but we don't know what that means yet," Small said. "What I suspect is that it's not just the scan. There may be a lot of other variables that come into play that determine whether there's a close correlation between the tau deposits in the brain and any clinical symptoms."