The field OPI Unit relies on a calculation of Current Avg Price - Baseline Avg price. This field is null when either of the two fields are set to null. so for instance - 0A15SY - has a current avg price of 207 with a baseline avg price of null. When using the non aggregate calculation - the value output is null. The row for Prod number 7281 has no average price value so the field is coming back as null for Contro Valve.
Once the values are all modified with ZN() so OPI Unit appears as - ZN([Current Avarage Price]) - zn([Baseline Avarage Price]) then the values expressed are a result of filling in all of those blanks with zero. This fixes the Contro Valve numbers apparently but I'm not sure why they would be incorrect for Setpoint with this fied. I show the OPI $ value in this case would be 1,871,765
I hope that helps explain whats going on so you can determine what the correct calculations should be.