interval type additional option

Hi

I have a value '400 days 30 sec':: interval

I don't understand why cast

postgres=# select '400days 30sec'::interval minute to second;
+-------------------+
| interval |
+-------------------+
| 400 days 00:00:30 |
+-------------------+
(1 row)

produce 400 days

or

postgres=# create table foo(a interval minute to second);
CREATE TABLE
postgres=# insert into foo values('400 days');
INSERT 0 1
postgres=# select * from foo;
+----------+
| a |
+----------+
| 400 days |
+----------+
(1 row)

What is sense of this feature?

Regards

Pavel