The new music filter shows the top [N] songs with the newest timestamps. For example if you're adding 5 files, if they have newer timestamps than the others in the list these should displace 5 files with the oldest timestamps. You can adjust the number of songs [N] shown in this list in settings>appearance.
This is probably not the way some of you would expect this to work, but showing the songs most recently added to the database would require us to keep a historical copy of the database to compare against. This would severly slow down database builds, so we decided against it and to just rebuild a fresh database every time changes are found.