those guests could very easily be search engine bots
Googlebot & MSNbot typically spawn the same site multiple times to increase the logging speed

The site admin should be able to see if any of the guests have consecutive IP's

I've banned bots in the by-gone days of my forum admin duties due to bandwidth issues

*edit*
From memory, there's a mod on the phpbb site to show bots by their names, as opposed to just "guest"