Actual AF vs target AF in Introducing artificial mutation SNV in bam (biostar404363)

[ericaenjoy3]

I used the tool biostar404363 to introduce artificial mutation SNV in bam, and the result I had (enclosed) showed actual AF tend to be lower than target AF at all ranges of depth coverage (low: DP < 20; medium: 20 <= DP < 80; high: 80 <= DP < 100; ultra high: 100 <= DP). I wonder if there is any known explanation(s) for this? Many thanks!