FBDI Net Change uploads in supply planning,Why do some records update while other create new rows?

Summary:

We are using FBDI Templates to upload master data (Such as BOMs and Routings) into Oracle fusion supply planning.During uploads, we run data collection i NET CHANGE mode.
When we make changes to existing records in the FBDI template and re-upload the file. in most cases the system updates the existing data correctly .However, in some scenarios instead of updating the existing record it creates new rows for the same item, BOM or Routing.
Questions:

1.Why fusion sometimes creates new records instead of updating existing ones?
2.As there specific fields such as Effective start or end Dates,Alternate BOM designator that when changed cause creation of new rows?