Deleting save impossible

When deleting a save, it doesn't show in the list any more.
But when creating a new save with the exact same name, you are on the deleted world (buildings still here...)
After looking more in detail in the saves folder, here is what I found:
even after deleting the save (ingame), the folder is still in the saves folder with this content:
-region
-stats
-level.dat
-session.lock