Three reasons can explain this:
- This Issues Drilldown service only shows added issues, not fixed ones. Therefore the number you see in this service can be different from the one displayed on a dashboard through the Rules Compliance widget (that displays added issues - fixed issues).
- The algorithm detecting new issues is very good but still perfectible. Therefore new issues can sometimes appear only because SonarQubeTM did SonarQubedid not recognize it already existed.
- You are looking at a period of X days which goes beyond the first analysis made after migration to version 2.5. In this case there can be a discrepancy due to the fact that prior to 2.5, data to build this service were not collected. This issue will disappear as soon as period does not go beyond first analysis.