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The active user count is done in SiteStats.php, the query is 

                $activeUsers = $dbr->selectField( 'recentchanges', 'COUNT( DISTINCT rc_user_text )',
                        array( 'rc_user != 0', 'rc_bot' => 0, "rc_log_type != 'newusers'" ),
                        __METHOD__ );

I saw that it showed 6 users, and I could count more than that myself in RC, so I went to the toolserver to check it out.

mysql> select rc_user_text, rc_title, rc_log_type, rc_user, rc_bot from recentchanges where rc_title='πορτραίτο';
| Francostas   | πορτραίτο | NULL        |     801 |      0 |

there you can see that rc_log_type = NULL

mysql> select  DISTINCT rc_user_text  from recentchanges where rc_user != 0 and rc_bot=0 and rc_log_type != 'newusers';
Francostas is not in the list, only 6 users are returned

mysql> select  DISTINCT rc_user_text  from recentchanges where rc_user != 0 and rc_bot=0;
this does return his name.  

this is because... 
mysql> select NULL != 'newuser';
|             NULL | 

Maybe the rc_log_type should be an empty string?