Full System scanned only a fraction of files

NIS 2011 v 18.7.0.12

Vista Home Premium 32 bit

Vista service pack 2

 

I run a daily full system scan 3am

 

Normally when I wake up, I will see that 800,000+ files were scanned. This morning I woke up and found that  the scan "completed" but only 235,377 files were scanned.

 

Just uesterday the full scan showed completed and 834,975 files were scanned.

 

any ideas what may have happened?

My secureity icon shows a green check mark