Closed:Difference between samtools unmapped reads and picard metrics file unmapped reads

Question

[DVA]

I have noticed that the count of unmapped reads from:

samtools view -f 4 $sample.sam > $sample.unmapped.sam

is LESS than (about ~90% of) what I got from the "UNMAPPED_READS" column in the metrics file generated by MarkDuplicates via picard:

java -Xmx45g -jar /dir/picard.jar MarkDuplicates INPUT=$sample.sam OUTPUT=$sample.sort.mdup.bam METRICS_FILE=$sample.metrics.mdup.txt

I checked picard definition of UNMAPPED_READS, and it is: "The total number of unmapped reads examined. (Primary, non-supplemental)". I don't understand why the two numbers do not match. Any suggestion please? Thank you.