Confusing "star" behavior in UNION ALL operator

{ "a1": 1 ,    "b1" : 1}
{ "a1": 2 ,    "b1" : 1}
{ "a1": 2 ,    "b1" : 2}
{ "a1": 3 ,    "b1" : 2}
{ "a1": null , "b1" : 3}

0: jdbc:drill:schema=dfs> select * from `t1.json` union all select * from `t1.json`;
+------------+------------+
|     a1     |     b1     |
+------------+------------+
| 1          | 1          |
| 2          | 1          |
| 2          | 2          |
| 3          | 2          |
| null       | 3          |
| 1          | 1          |
| 2          | 1          |
| 2          | 2          |
| 3          | 2          |
| null       | 3          |
+------------+------------+
10 rows selected (0.126 seconds)

0: jdbc:drill:schema=dfs> select a1, b1 from `t1.json` union all select * from `t1.json`;
Query failed: Query failed: Failure validating SQL. org.eigenbase.util.EigenbaseContextException: At line 1, column 47: Column count mismatch in UNION ALL
Error: exception while executing query: Failure while executing query. (state=,code=0)

0: jdbc:drill:schema=dfs> select a1 from `t1.json` union all select * from `t1.json`;
+------------+
|     a1     |
+------------+
| 1          |
| 2          |
| 2          |
| 3          |
| null       |
| 1          |
| 2          |
| 2          |
| 3          |
| null       |
+------------+
10 rows selected (0.111 seconds)