Voltage Sensor values change using thingspeak

when I don't include thingspeak in the code line the value of a voltage sensor is accurately read in the serial monitor, but when I start the code with thingspeak the value that reads is inaccurate and read by the serial moitor is also inaccurate, the value is not accurate only when installing thingspeak, whether this incident likes to happen please solution

#include <Filters.h>
#include <SPI.h>
#include <Wire.h>
#include <Ethernet.h>
#include <ThingSpeak.h>


float testFrequency = 50; 
float windowLength = 40.0/testFrequency;

//tegangan
int Sensor = 0;
float intercept = -1;
float slope = 1.1833;
float current_Volts;

unsigned long printPeriod = 1000; //Refresh rate
unsigned long previousMillis = 0;


//Ethernet
byte mac[] = {
  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED
};
EthernetClient client;


//thingspeak
String apikey = "xxxxxxxxxxxxx";
unsigned long myChanelNumber = xxxxxxxxxxx;
const char * myWriteAPIKey = "xxxxxxxxxxxx";


void setup() {
  Serial.begin(9600);

  //tes ethernet
  //jaringan
  if  (Ethernet.begin(mac) == 0) {
      Serial.println("Failed to configure Ethernet using DHCP");
      for (;;);
      }
  Serial.print("My IP Address: ");
  Serial.println(Ethernet.localIP());
  delay(1000);
  Serial.println("");
  
  ThingSpeak.begin(client);

}

void loop() {
  
  RunningStatistics inputStatsV;
  inputStatsV.setWindowSecs( windowLength );
  
  while( true ) {
    Sensor = analogRead(A0);
    inputStatsV.input(Sensor);

    if((unsigned long)(millis() - previousMillis) >= printPeriod) { 
      previousMillis = millis();
      
      //tegangan
      current_Volts = intercept + slope * inputStatsV.sigma();

      //tegangan
      Serial.print(" Voltage :  ");
      Serial.println( current_Volts );
    
    ThingSpeak.writeField(myChanelNumber, 1, current_Volts, myWriteAPIKey );
    
    }   
  }
}