Imagine you are going to a site with firefox, and you go back to that site later on safari. At the moment, there is no way for the platform to know that all those actions are from the same user. You'll have two user points: one with the user agent ID from firefox and the other with the user agent ID from safari.
If later you log into the site with your user account on both navigators, the two user points will be associated with a common user account ID identifier. The platform will automatically merge the two user points and their activities to represent the reality that we now know those two users were the same.