I've noticed that people who have their "keep user logged in" option set for some ungodly high count, like 30 minutes tend to have a TON more guests. If you notice, the slurps and spiders tend to use different IP addresses. People who have their forums set to super long keep user active variables are full of slurps and spiders as guests.
I guess they think it makes the forum look more active.
My guest count is NEVER higher then my logged in user account. Usually 200-250 users logged in, 60-80 guests