Why do we add issue_closed_time - issue_created_time for closed issues?

[mlabeeb03]

As you can see here:
https://github.com/github/issue-metrics/blob/main/labels.py#L86-L90
If the issue is closed we add the entire time the issue was open to the time the label was assigned to an item. The seems wrong to me for example if:

issue created on day 0
label assigned on day 5
label unassigned on day 10
issue closed on day 15

The answer for that label should be 5 but according to this code it is going to be 20 since we add issue_closed_time - issue_created_time to the label time if the issue is closed.
Can someone let me know if I missed something?