It requests the results on line 504 below, then caches a list of user names from the user IDs returned in the report result data, but it has to then make a second request to get the exact same set of results in order to display them. It does this because a recordset implements PHP's iterator interface but doesn't implement the rewind method.
 
https://github.com/moodle/moodle/blob/master/report/log/classes/table_log.php#L504
 
So the call to $this->update_users_used(); iterates through the results but has no option other than to close the recordset, which means a new one has to be created in order for the results to be displayed.