small discrepancy in mapping stats #39

For a given sample I have the 2,636,688 in the mapping_stats file for the number of
UniqueFWDandREV which equals the number of UniqueFWDorREV.

However if I run the ngsutils command
bamutils filter -mapped -eq NH:i 1

on the corresponding bam file, the reported number of retained alignments is
so 5273384, thus 2636692 unique aligners (this is paired end).