Problem with encoder readings with arduino

we are getting the encoder readings from the vesc using basic arduino but the issue is that the readings or anaolg value for revolution is varying in two directions..i.e for  example  if the motor is rotating right for 10sec the encoder reading is 200 and if the motor is rotating left for 10sec the encoder reading is somwere around 300 or 100 .its is keep on varying but not a constant value.What is actual problem??? Can anyone solve this out??