Incomplete user statistics: I have installed the GuiXT user statistics as described in your documentation http://www.synactive.com/docu_e/doc_statistics.html.

Why do only some of the users appear in the statistics report, and not all of them?

Most likely the users that you do not see are those still using a GuiXT version prior to 2011 Q1 4.
The statistics feature is not supported in the earlier versions.