I have recently noticed that the suggested topic list at the end of a topic behaves quite differently depending on if the user is logged in or visiting as guest:
1. Suggested topics when logged in shows a set of topics not older than 5 days
2. Suggested topics when visiting the exact same topic as guest, shows mostly old topics
As site admin you can set how old topics can be for being included in the suggested topic list, but as it seems, that setting is only honored for logged in users?