To determine the HistoryNumber, only the number of AttestationRuns available (for this policy) is currently determined. However, if you use the feature to automatically delete old AttestationCases, the number of AttestationRuns remains constant after the defined limit has been reached and therefore the HistoryNumber is identical for each further AttestationRun.
This is a product defect (31373).
This will be fixed in a future release of the product. If you require this immediately corrected, please contact Support for a hotfix referencing the defect ID 31373.