Because they are not deleted as you thought.
Files in your device are stored using index structure. Index structure is like the catalog in a book. Device can find a file quickly using the catalog. When we delete a file, device only delete the index so that the file cannot be found anymore. The file itself, however, is still there.
That is why it takes so long to copy or move a file but only an instant to delete one. The file is just only marked as "deleted" but not actually deleted.
So it is possible that those deleted files can be recovered by other means. And Dr.Fone can provide you a solution to erase data permanently.
How can Dr.Fone erase data permanently?
First of all, Dr.Fone will erase the real files in your device, not just index.
Moreover, after erase the file itself, Dr.Fone will fill your device storage with random data to overwrite deleted files, then erase and fill again until there is no chance of recovery. Military grade algorithm USDo.5220 is used to erase and even FBI cannot recover erased device.