They might have been part of playlists you deleted or they might be tracks you tried to replace.

Or they might be duplicates of tracks you already have in playlists.

Or it might be a mixture of all of the above.

Bitt's point is... you have to find out what the tracks are and why they're there. And compare them to what's actually in your playlists. And decide, on a case-by-case basis, whether or not to add them to a playlist or to delete them.