the playlist owner updates their playlist as they see fit, if you prefer not to have changes to a particular playlist you should save it as your own.
It alos depends on the playlist; things like the 'Songs from the Ads' playlist will by nature get more tracks added as more adverts are on.... However, playlists such as 'Throwback Thursday' is a complete re-write of the playlist.
Please use the search feature before posting a problem/bug/idea. What's a Spotify Rock Star, and how do I become one? Using Spotify Since March 10th 2009 Twitter: @mechanimal82