Characters can be created in a number of ways: GRSS Importing, Armory importing, and Uploading your roster are the primary mechanisms that typically create characters without associating them.

Alright, different but related question:

Why do some accounts not have any displayed Last Accessed date? I know some of them have logged in before.

It's possible, however unlikely, that they may have only ever logged in during crazy peek times when I disabled that tracking. The architecture has been changed since, such that we won't need to do that anymore, but it is possible.

It's also possible that they created their account but never actually logged in (they just visit the site, without logging in).

That's the only way that an account can be lacking a "Last Accessed Date"