I encountered this problem a number of times. I believe, in my case at least, that I noticed a pattern. I believe that facebook's activity log filters posts related to disabled accounts.

I think that when someone decides to re-enable their disabled account, your posts related to their account re-appear in your activity log. Therefore, one can never be sure if all activity is deleted, and one must iteratively return to the activity log every so often to delete re-appearing posts.