You are noticing the business services on Event Management Dashboard are showing wrong impact and are out of sync with the alerts.
If you tried cleaning up the records in em_alert and em_alert_history tables, then with the new alerts coming in, certain business services might be showing wrong impact calculation.
This is because there are multiple other places where we do the impact calculation from. Deleting em_alert and em_alert_history would not reset the impact calculations on business services.
In the EM dashboard, for the services where the impact is not showing correctly and out of sync, moving them to non-operational and then setting back to operational, will trigger fresh impact calculation jobs. This will set back the services to sync.