It is designed to rotate content. The interval of rotation depends on how much “valuable” content you don’t have already on your phone. Things like listening to stuff, favoriting stuff, adding stuff all impact an item’s ranking.
The idea is that SDL are an ever rotating chef’s menu of your own music you’re likely to enjoy. If you really enjoy it, “saving” an item ensures it won’t get scooped up and deleted in the next rotation.
Hope this helps clarify!
Thanks for testing 