Site search that matches in more than one field
foo bar implies foo AND bar by default.  What you're probably looking for is foo OR bar.

If you look at craft_searchindex, you can see that keywords for fields are indexed on a per field basis, which is why you'd need the OR to search across multiple fields.

You can omit OR if you already know the field names ahead of time, though. field1:foo field2:bar.

You can see more search syntax options here.