It may be in the configuration of the different scans.
The on-access scanning (McShield) has completely different settings than the scheduled scans.
I would compare the two policies and evaluate the differences
If both the policies are identical:
My opinion is this:
Either your on-access scanner is set to scan way too much
or
Your Scheduled full scan is set way too light.
Back to the issue..
If they are the same..
You do realize that the on-access scanner will only evaluate the "virus" when it is copied, accesses, moved etc..
The Scheduled scan will detect the "virus" as it sits on the machine.
Other than a policy mis-match - This would be strange behaviour as they use the same DAT file for scanning.