A large set of trolls on web forums are ordinary members who creates an alias account and takes advantage of their knowledge about the other members to write optimized troll posts.

In some situations, a forum could try to catch multiple accounts from the same IP (possibly ok if multiple people behind a firewall) and potentially flag low post-count accounts that shares the same IP with someone else.

On the Keil forum, it feels like half the members don't want their names known or are posting under more than one name.