How does Google Trends calculate results

“Football” in category sport has more searches than “football” in search term. To me, it doesn’t make sense. “Football” in sport should be included in “football” search term. How does Google even know in what category should they put a search when someone types in “football”?
Another thing: is for example “chess online” search included in “chess”? It would be helpful if it was, although not sure if it is feasible from Google’s perspective to include all exceptions like that.

Thank you in advance for help.