BigQuery union behavior with different column definition

uniontest.sql

create table `uniontest.uniontest_1`
  (partition_key date, cluster_key string, hoge string)
  partition by partition_key
  cluster by cluster_key
  as select cast('2020-06-22' as date), 'a', 'b';

create table `uniontest.uniontest_2`
  (partition_key date, cluster_key string, piyo string)
  partition by partition_key
  cluster by cluster_key
  as select cast('2020-06-23' as date), 'c', 'd';
  
-- Now, uniontest_1 is:
-- +---------------+-------------+------+
-- | partition_key | cluster_key | hoge |
-- +---------------+-------------+------+
-- |    2020-06-22 | a           | b    |
-- +---------------+-------------+------+
-- uniontest_2 is: 
-- +---------------+-------------+------+
-- | partition_key | cluster_key | piyo |
-- +---------------+-------------+------+
-- |    2020-06-23 | c           | d    |
-- +---------------+-------------+------+

select *
from `uniontest.uniontest_*`
-- This query returns:
-- +---------------+-------------+------+
-- | partition_key | cluster_key | piyo |
-- +---------------+-------------+------+
-- |    2020-06-22 | a           | NULL |
-- |    2020-06-23 | c           | d    |
-- +---------------+-------------+------+