When you want to do an on-demand scan you launch the Simple User Interface. Before that starts it scans all programs, files, etc. that are loaded into your system memory (RAM).
If it finds a virus in the memory it will suggest that you do a boot-time scan (before windows starts and loads files into memory).
If there was something in memory then it would still be there when you ran the on-demand scan, that may be hiding other malware, that would make the scan of less worth.