The addition could result in one issue being counted multiple times, e.g. if an issue was committed and removed from the Sprint, it will be counted both times. Is that what you are looking for?
How do you determine that other issue type issues are accounted for in the measure. Judging from the formula and page filters other issue types should not be included. Try drilling through issues for that particular calculated measure and verify the issues and what makes up the total.
If you require the unique count of issues that were committed, added and/or removed, a more complex calculation will be required. Let me know if you need something like that.