I'm a bit puzzled about this one too, but looking at report/stats/locallib.php this data is based of the relevant moodle stats tables (daily, weekly etc). In those tables the roleid is used, and roleid of 6 for example is a guest (depends on your install). In the UI, you'll also notice the filter is by "All Activity (all roles)".
So what I think the "Guest" column is saying is this was the role of the user when they were looking at a particular course. I suppose anyone that isn't enrolled in that course would be considered to have a guest role. If users can view courses they aren't enrolled in easily then I can see how these numbers would be quite high. Or alternatively, perhaps there is a user that is performing actions on courses (e.g. an admin) that isn't enrolled in courses it is performing the actions on? Check an individual unit and look at the logs to see if the user is in fact enrolled in the course.