Unrecoverable files? Options

[SubnetMask]

I just tried Recuva and ran across something I don't understand. The drive I'm recovering files off of was used to back up data, then formatted when the data move requiring the backup was done, and put on a shelf. Running Recuva found pretty much everything that was there. The strange thing, though, is that there are a bunch of files marked as "Unrecoverable", and "No Overwritten Clusters Detected", and when I tried to recover several, they seemed to recover fine. I can understand a file being unrecoverable when there are overwritten sectors, but if there are no overwritten sectors, why would the file be marked as "unrecoverable"?