stxkind only explain two elements. left two unexplained.

 stxkind
---------
 {d}
 {m}
 {f}
 {e}
(4 rows)

create statistics flight_expr1 on(
    extract
    (month from scheduled_departure at time zone 'Asia/Kolkata')
    ) from flights;

select stxkind from pg_statistic_ext where stxname =  'flight_expr';