Less files than usual and takes less time

Hello today i did two antivirus scans one for just usual (i usually scan once every week) and the second one just because i wanted to. On the second scan it scanned less files than usual and took about 10 minutes rather than the usual 20-30 minutes. It usually scans about 301,000 files but the second scan only scanned around 290,000. Is norton skipping things?