See PHI66. See that issue for context. This UI is bad broken legacy junk, but was especially broken when reporting merges.
These do not currently generate a "status" transaction, so they were never counted as task closures. Pretend they're normal closures.
This is still wrong, but should be much closer to the real numbers. Specifically, if you merge a closed task into another task, it will incorrectly be counted as an extra close. This could result in negative tasks, but the numbers should be much closer to reality than they are today even so.
The "Facts" application (T1562) is the real pathway forward here in the longer term.