I don't use that database myself, but according to this documentation, all joins in SQLite are based on the cartesian product of the left and right tables.
NATURAL
joins use common column names as "keys" to combined two tables. Using parentheses forces a "derived table" to be built before the outer join is processed. Without parentheses, it does not "see" the common column names, so the second NATURAL
keyword is ignored.
A little advice: it's good to understand the mechanics of how a NATURAL JOIN
works in case you ever see it in code, but do not use it yourself. It is a "dangerous" construct with little value. Just my opinion.