Grafana Funnel panel - Doubts to be clarified

Funnel grafana Dashboard shows wrongly S1 - Percentage Increase as -33.3. Actualy it should be displayed as -30. If i change the Stage-Lead Actual value from 1300 to 1200, Dashboard shows correctly as -20. But if the Stage-Lead Actual value is 1300 as below table, Dashboard shows wrongly S1 - Percentage Increase as -33.3. Please solve this issue. what is the logic behind?