Three duplicate-filter rows separate raw reads from the unique count used downstream. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Duplicate filtering is subtraction, not a label
The low row keeps 20 raw reads minus 4 duplicates, so unique reads are 16.
20−4=16
Different raw rows can land on the same unique count
Raw 28 with 4 duplicates and raw 30 with 6 duplicates both give 24 unique reads.
28−4=2430−6=24
Coverage must use unique reads
The accepted coverage row later uses 24 unique reads, not the raw count 30.