Assembly Failure Rate Calculation not equal to sum of parts

I am running a 217Plus analysis of an assembly with 3 subassemblies. When the calculation completes, Windchill presents a total failure rate ("Failure Rate, Predicted" attribute). That total is not equal to the total of the parts listed below each subassembly. When I run the exact same project as a 217F prediction, the total failure rate is equal to the sum of the parts listed in each subassembly. Why is this different? What is the correct value to use for the 217Plus overall predicted failure rate?