User can not use links after send out

Got bug from client today.
After send out user were not able to open any links in the message.
After little bit of research i've noticed that url contains some unique per each sendout guids like www.xxxxxx.co.nz/92fab72e-cc9b-4171-b5c5-4317d977353a

Every subscriber would have these tokens different 'sf='. I noticed that bug happend to subscribers, which were created in code by NewslettersManager.GetManager().CreateSubscriber(false, subscriberId);

When i checked sitefinity table  sf_subscriber i found out that all users which could not use links had short_id null.

My fix was to delete all subscribers with shot_id = null and import them again with sitefinity import option.

What does short_id means and why in documentation example create subscriber it has 'false' for short_id. 

if (mailingList != null)            
Subscriber subscriber = manager.CreateSubscriber(false, subscriberId);        
subscriber.FirstName = firstName;        
subscriber.LastName = lastName;        
subscriber.Email = email;         
manager.Subscribe(subscriber, mailingListId);        manager.SaveChanges();