Truthful short answer: I really don't have any special insights into where/ how that site is deriving their figures.
I follow another stock much more closely than AVXL and I ran across the sight while doing research. The CNBC site showed a large institutional ownership ( in the other company) that other sites including the Nasdaq site did not show. Somehow their site seems to be more accurate than many others I have used. It is nothing more than my opinion though, that their figures are timely and accurate. I certainly welcome any fact based criticism or further due diligence anyone wants to provide into how that turnover figure could be wrong or misleading.