Do you have any buyers buying more than one item, if so if a packet containing 3 items was shown as late it would only count as one.
My figures are also different, but no defects just 2 late delivery, and I have several buyers buying more than one item, which could explain difference between number of transactions and number on which late delivery is calculated.
For example a packet containing 3 items arrives late. Would be rather unfair to get 3 lates.