I repeatedly try to check some files in green (excellent condition) and do a secure delete checked. They would go red with status changed to unrecoverable. But then when I rescan the same drive the files would show up again as green. I tested recovering them and indeed they all were recovered in prestine condition. Are there any tricks about file properties or other issues that would cause this to happen?

I've found that the file status coulmn is not gospel, as we westerners tend to say. Have a look at the file info panel. Has the file been overwritten by another file? If it has then you may be looking at the overwriting file, and recovering that. Secure deleting an overwritten file does not touch the contents of the overwriting file.