1. You've securely deleted a file, but you can still see the Preview or Header information.
This usually occurs when the file you've securely deleted has already been overwritten with another file. Examine the the Preview to determine if the file is really the one you securely deleted. The Header tab, which displays the first 128 bytes of the file, can be misleading, since the contents of the Header tab may not change if the first cluster has already been overwritten with another file.
2. Some files that have been securely deleted are marked as recoverable.
If you have used Recuva (or any other similar app) to erase files, the files may still be marked as being recoverable, but they are not. The MFT entry may still exist, but the contents of the file are unreadable.
3. I want to recover a file that has been securely
Unfortunately, this isn't possible. Once it's gone, it's gone.