Conversion counting discrepancies
If an advertiser uses a third-party conversion tracking service, they might find discrepancies between the number of conversions counted by DFP and by the third-party service.
Each third-party counting service uses its own method for counting conversions. Because of differences among services, discrepancies are inevitable.
If your activity is counting unique users within each 24-hour day, it's possible that a third-party service will count multiple conversions for a single user, while DFP will count that user only once per day.