That could be why. But there could be a potential issue.

1) User who forgot their password before they activate are stuck.
Workaround:
When they provide the right username & password combination but wrong confirmation code, have the error message such as
- error message: Invalid username and password combination, click here to reset your password or wait x hours to register again.
- solution: Make it posibble to reset password before activation. Gives clear instruction how to proceed if you do not offer password reset before activation. (i.e, let them know how long to wait before they can register using same email & username)

When they provide invalid or expired confirmation code, have the error message such as
- error message: Invalid activation key or signup has expired. You may register using the same email again.
- solution: Let them know more clearly what can they do in this case. If your database still store their info, offer a link to resend new activation code. If not, let them know they should register again.

In my case, I'm not sure if I have my username/password combination incorrect. I have not activated the account yet and when I tried to sign in, I get the following error:
Invalid activation key or signup has expired.
I don't think the signup expired yet since it's only been a short while. I also got the activation code from email. So, I think I should havve wrong credential info. The system should be clear on this.

Please let me know how can I proceed now. Thanks!