Oracle Database Discussions

Please,
working on Oracle 10g realease 2 on windows, I'm testing switch over on dataguard configuration.

Step1: on Primary database

SQL> ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY;
ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY
*
ERROR at line 1:
ORA-16139: media recovery required

I found that the error above, means all redo data on standby were not recovered.

Step 2: I verify if redo needed to be apply on stand by
So on standby database I did:

SQL> SELECT SEQUENCE#,APPLIED FROM V$ARCHIVED_LOG ORDER BY SEQUENCE#;

SEQUENCE# APP
---------- ---
3 YES
4 YES
5 YES
6 YES
7 YES
8 YES
9 YES
10 YES
11 YES
12 YES
13 YES
This output shows that all redo were applied in the standby database.
Step 3: on primary database I did
SQL> select open_mode from v$database;

OPEN_MODE
----------
MOUNTED
the primary database is now in mount state, this means both primary and standby database are now in mount state.

Step4: on primary database I did again
SQL> ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY;
Database altered.

My Question is: Why on step 1 I got the error eventhough the step 2 shows that all redo data were already applied on standby?

There is an issue with the step 4, done on primary database eventhough the step1 on the same primary database failed?
Thanks ,