gnupg secret key vs session key

[Geoffrey Leach]

My use of gnupg is very simple (or so I thought)

I created a user (defaults accepted) with a nice long passphrase
I then encrypted a file using --symmetric
I decrypt the file

%gpg -d ...
You need a passphrase to unlock the secret key for ...
<enter the secret key from user creation>
gpg: Invalid passphrase; please try again ...
two more attempts, same result
gpg: 3DES encrypted session key
Enter passphrase:
entering the passphrase created above works fine.

I understand that secret key is different from session key, but I don't 
understand why the passphrase is not acceptable in the first case, but 
is in the last.