Cannot Detect OS reliably

["Michael Condon" <admin () singulartechnologysolutions com>]

I am running a scan nmap -T Aggressive -A -v <target>
It returns the message OSScan results may be unreliable because we could not find at least 1 open and 1 closed port.
Is there any remedy for this, or does the target have itself hardened too much to prevent this?
The Unix & Win32 report very different guesses on the OS.
Nessus uses a remote HTTP Banner to guess the OS, which comes up with yet another result.

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