How can I determine why the pre-populated Apple ID is not working?

When I first go to the Apple site to log in, the screen pre-populates with what appears to be my correct Apple ID, my email address, but if I then enter my password, I get an error.

BUT if I select "use a different Apple ID", and then enter my email address exactly as it first appeared, I can then use my password and successfully log in.

How can I determine WHY the pre-populated Apple ID is not working?

[Re-Titled by Moderator]