Track iPhone data usage

Hello I am trying to to track phone data usage (both Wi-Fi and Cellular). I am using the following code to get device data usage https://stackoverflow.com/questions/25888272/track-cellular-data-usage-using-swift as I understand it is the only way to get it without having to use private API's.

The problem is I can't figure out why the usage seems to be high compared to other apps that track data usage. I was wondering if anyone could provide documentation on the interfaces used on the code from stack overflow or how to get access to more detailed data usage.

What I am doing with the code from stack overflow is take the total Wi-Fi and cellular usage and compare it with the new one every 15 minutes to store a data usage event.

public func calculateTempDataUsageEvent() -> Void {
        
        let totalWifiDataUsage = Int64(wifiNetworkInfo.getWifiDataUsage())
        let totalCellularDataUsage = Int64(cellularNetworkInfo.getCellularDataUsage())
        let firstTimestamp = Date().millisecondsSince1970

        let tempDataUsageEvent : TempDataUsageEvent
        let phoneDataUsage : PhoneDataUsage
        
        let realm = try! Realm()
        
        //Check if realm for PhoneDataUsage is empty, if this is the case store initial phone data usage so we can keep track of it's changes
        if(realm.objects(PhoneDataUsage.self).isEmpty) {

            phoneDataUsage = PhoneDataUsage(timestamp: firstTimestamp, offsetMinutes: Int32(TimeZone.current.secondsFromGMT() * 1000), totalCellularDataUsage: totalCellularDataUsage, totalWifiDataUsage: totalWifiDataUsage)
            
            do {
                try realm.write {
                    realm.add(phoneDataUsage)
                    print("Stored Phone Data Usage in Realm")
                }
                
            } catch let error as NSError {
                print("Failed to store Phone Data Usage in Realm with \(error)")
            }
            
            return
            
        } else {
            
            //TODO: Handle this unwrap in a better way
            let lastPhoneDataUsage : PhoneDataUsage = getLastPhoneDataUsage()!
            let tempEndTimestamp : Int64 = Date().millisecondsSince1970
            let tempEventDuration : Int32 = Int32(tempEndTimestamp - lastPhoneDataUsage.timestamp)
            let tempOffsetMinutes : Int32 = Int32(TimeZone.current.secondsFromGMT() * 1000)
            
            //Handle phone restart
            //TODO: maybe need to know phone uptime possible issue time zone change? https://developer.apple.com/forums/thread/101874
            
            var tempEventWifiDataUsage : Int64
            var tempEventCellularDataUsage : Int64
            
            if(totalWifiDataUsage < lastPhoneDataUsage.totalWifiDataUsage || totalCellularDataUsage < lastPhoneDataUsage.totalCellularDataUsage) {

                if(totalWifiDataUsage < lastPhoneDataUsage.totalWifiDataUsage) {
                    tempEventWifiDataUsage = totalWifiDataUsage
                } else {
                    tempEventWifiDataUsage = totalWifiDataUsage - lastPhoneDataUsage.totalWifiDataUsage
                }
                
                if(totalCellularDataUsage < lastPhoneDataUsage.totalCellularDataUsage) {
                    tempEventCellularDataUsage = totalCellularDataUsage
                } else {
                    tempEventCellularDataUsage = totalCellularDataUsage - lastPhoneDataUsage.totalCellularDataUsage
                }
                
                phoneDataUsage = PhoneDataUsage(timestamp: tempEndTimestamp, offsetMinutes: tempOffsetMinutes, totalCellularDataUsage: totalCellularDataUsage, totalWifiDataUsage: totalWifiDataUsage)
                
                tempDataUsageEvent = TempDataUsageEvent(startTimestamp: lastPhoneDataUsage.timestamp, endTimestamp: tempEndTimestamp, offsetMinutes: tempOffsetMinutes, eventDuration: tempEventDuration, eventCellularDataUsage: tempEventCellularDataUsage, eventWifiDataUsage: tempEventWifiDataUsage, sendToBackend: true)
         
            } else {
                
                //Default case where phone has not rebooted and API not reset
                tempEventWifiDataUsage = totalWifiDataUsage - lastPhoneDataUsage.totalWifiDataUsage
                tempEventCellularDataUsage = totalCellularDataUsage - lastPhoneDataUsage.totalCellularDataUsage
                
                phoneDataUsage = PhoneDataUsage(timestamp: tempEndTimestamp, offsetMinutes: tempOffsetMinutes, totalCellularDataUsage: totalCellularDataUsage, totalWifiDataUsage: totalWifiDataUsage)
                
                tempDataUsageEvent = TempDataUsageEvent(startTimestamp: lastPhoneDataUsage.timestamp, endTimestamp: tempEndTimestamp, offsetMinutes: tempOffsetMinutes, eventDuration: tempEventDuration, eventCellularDataUsage: tempEventCellularDataUsage, eventWifiDataUsage: tempEventWifiDataUsage, sendToBackend: true)
            }
            
        }
        
        //Write to Realm
        do {
            try realm.write {
                realm.add(tempDataUsageEvent)
                realm.add(phoneDataUsage)
                print("Stored Temp Data Usage event in Realm")
            }
            
        } catch let error as NSError {
            print("Failed to store Temp Data Usage event in Realm with \(error)")
        }
        
       

I called temporal data usage as I then use a 'legacy' table to store all usage and temporal data usage is deleted. I do this to know what is not on backend. Finally I add the values from the events and sum them to show data usage.

public func getTempDataUsageEventSum() -> (wifiDataUsageSum : Int64, cellularDataUsageSum: Int64) {
        /**
        Returns Wi-Fi/Cell total data usage from TempDataUsageEvent Realm, 0 if empty
         */
        
        let realm = try! Realm()
        
        let totalTempWifiDataUsage: Int64 = realm.objects(TempDataUsageEvent.self).sum(ofProperty: "eventWifiDataUsage")
        let totalTempCellularDataUsage : Int64 = realm.objects(TempDataUsageEvent.self).sum(ofProperty: "eventCellularDataUsage")
        
        return (totalTempWifiDataUsage, totalTempCellularDataUsage)
    }