I thought this is quite funny, looking at user106011 profile:
I've found quite a similar question here, with the answer
Seen means the last interval check that was made
But there was only a matter of some minutes difference, while here, it's more than one year!
And the answer to this question: Account “last activity time” is not always updated states that:
Seen: equivalent to LastAccessDate. Activity threshold 30 minutes
So, is this a bug or not? Who can solve the mystery? :)