Non-Correlated Subqueries III

I am confused as to why ‘dep_date’ is required in this exercise:
https://www.codecademy.com/courses/sql-table-transformation/lessons/subqueries/exercises/non-correlated-iii?action=resume_content_item

I ended up asking for a solution after not being able to run the query. I seemed like the only difference was that the actual solution included dep_date in the subquery, which I didn’t include. I don’t understand why is it needed. Can anyone explain?

This is the code I used before seeing the solution:

SELECT a.dep_month, a.dep_day_of_week, avg(a.flight_distance) as average_distance
from (
  SELECT dep_month, dep_day_of_week, sum(distance) as flight_distance
  from flights
  group by 1,2,
  ) a
  group by 1,2;