How does Google Trends calculate results?

"Football" in the sports category has more research than "football" in the search term. For me, that does not make sense. "Football" in the sport should be included in the search term "football". How does Google even know in which category should it put a search when someone taps "football"?
Another thing: for example, is the search for "failures online" included in "failures"? This would be helpful, but I do not know if it is possible, from Google's point of view, to include all such exceptions.

Thank you in advance for help.