MariaDB [tpch1]> select 'Avg', avg(n_regionkey) from nation;
---------------------+

---------------------+

---------------------+
1 row in set (0.025 sec)

Notice there are four decimal places, which is correct.

Now see
MariaDB [tpch1]> select 'Max', max(n_regionkey) from nation
-> union
-> select 'Avg', avg(n_regionkey) from nation
-> ;
---------------------+

---------------------+

---------------------+
2 rows in set (0.023 sec)

Which puts 4 decimal places for both. Presumably, the MAX (an INT) is promoted to long double.

When used this way
select 'Avg', avg(n_regionkey) from nation
union
(select n_nationkey,n_name from nation where n_regionkey = 0) order by 1, 2;
----------------------+

----------------------+

----------------------+
6 rows in set (0.026 sec)

Which gives us the odd number of 9 decimals.

See working_tpch1_compareLogOnly/qa/DL_union.sql.ref.log.