The target is always an estimate at the time the broadcast was created because it doesn’t exclude contacts that are not sent the broadcast at the moment of sending for reasons such as:
They have already been sent it
They have unsubscribed
Their subscription has failed (multiple soft bounces or one hard bounce)
There is a setting in a broadcast that omits contacts whose subscriptions are all cancelled and whose subscriptions have all failed. Broadcasts will not be sent to these contacts.
Note: Failed sending may mean broadcast could not be sent because of an invalid email address. It may also mean broadcast wasn't sent because of host’s sending limit has been reached so the unsent messages will not be sent out anymore.