Funny detection behaviour

I've just captured about 2 dozen or so files from a clients PC which are about 300 - 500 kb each. I pre-scanned the USB Disk (right click Scan with ESET NOD32 Anti Virus) and it removed about half of them saying they were infected with Win32/Obsfugated....

Then i rescanned and it detected nothing. Then i went to transfer the files to my PC to a) submit to ESET and b) check when they have been added to the detections and it detected a few more as being infected. So why not the first time round

 

I have sometimes seen (rarely , though) things not detected by on-demand scanner but detected by AMON/eAMON . It might also be that you have something off for your on-demand scanner (e.g. AH off for your on-demand scan?)

 

Nothing off. All set to Strict Cleaning and boxes ticked......

 

What was detected actually ?

edit: trojan . Sorry! I saw it.