Why did a performance measure that compares the current and prior approved forecast show an unfavorable result when there is only one approved forecast?

If you generate KPI values when no previous approved forecast exists, a zero value is used to generate KPI values. So, the trend appears unfavorable because the difference between the current and prior forecast is the same as the current forecast.

The results will be accurate once a new forecast is approved, project performance data is summarized, and KPI values are generated.