User stories may have been added and removed more than once. The application calculates the total based on the unique line items either added or removed.
Note here that the Total Removed Story Points in the summary box is 2, yet the sum of the line items is 4. This is because story S139351 has been removed twice (also added twice but not shown in this screen shot). The net change in the story points is actually 2 since the story points removed are only being counted once against the iteration. The the story has been removed from the iteration twice and thus the count does show this value as 2.