It's possible that they are simply pushing the beta testing content data out to everyone to save time when they eventually decide to update the live game. That way they only have to change what is changed as a result of the test, which could be less than the whole game update.
Or they may simply have no choice since the content updates before you log in - they have to send the data to everyone, even if we don't get access to it.
Or they could be updating something else. We just don't know. As for me, my downloads are erratic in number of MBs.