I have noticed that the user creation timestamp is the time when the user is filling up the registration form.

Now suppose I registered on "1st Jan 2016" and I am validating my email ID next month i.e., on "1st Feb 2016". So elgg will show the user join date as 1st Jan, whereas technically it should be 1st Feb.

Also, the river activity for user join will add a post on 1st Jan (back dated)