Why Rework Variance is created in case of Transform Work Order

In case of Transform Work Order, Work in Process Job Close is ending up with accounted status as error. Create Accounting Execution Report shows below message

The input source values don't map to any output value defined in mapping set Rework Variance. Details: You must either define a default output for mapping set or specify mappings for all possible input values.

As per below Screenshot you can see Input cost=Output cost then how come Rework Variance is coming into picture.