Why some of the referring URLs are blank?

In the click details table the referrer field is filled only sometimes.
It is empty if:

1)    The user writes or copy-and-paste the tracking link directly in the browser URL field

2)    The browser or email client from which the click occurs is configured not to give this information

3)    Clicks are not coming from a human but from a robot

Related articles

• http://en.wikipedia.org/wiki/HTTP_referer