Reading this question How mark my question as answered on StackOverflow? I noticed that the avatar for both the user who asked the question and the user who provided the accepted answer have no links or gravatar.
It is a recent question, so what happened?
As the banner under the question says, it was migrated to Meta from Stack Overflow.
The users did not have accounts on Meta when the question was migrated, so there's nothing to link to. The person who answered has now registered here however, and the answer has been automatically linked to their account.
The users asked and answered the question while it was still on Stack Overflow. It was subsequently migrated to Meta where these users do not yet have an account.
The questions were likely migrated from other sites, and the users don't have accounts here (or they do but they're not linked).
This can also happen if a user's account has been expunged (either by request or by force).
