HealthKit Step Count Different From Activity App

Does anyone know why the count of steps for a specific day would be different than what the Activity app reports? For example on the 24th the Activity reports that I walked 12,304 steps. But when I run this code it reports that I walked 12,320 steps. A 16 step difference. I would think they should be identical. Any ideas?

Thanks!

let calendar = NSCalendar.current
let startDate = calendar.startOfDay(for: Date(dateString:"2017-12-24"))
let endDate = calendar.date(byAdding: .day, value: 1, to: startDate)!

QueryHelper.distanceForDates(startDate: startDate, endDate: endDate, type: .stepCount,
                                         unit: HKUnit.count(), completion: { (totalDistance, error) in
               guard let totalDistance = totalDistance else {
                  guard let error = error else { return }
                  print("Error retrieving data: \(error)")
                  return
                }
                print("The total number of steps is: \(totalDistance)")
            })

class func distanceForDates(startDate: Date, endDate: Date, type: HKQuantityTypeIdentifier, unit: HKUnit = HKUnit.count(),
                              completion: @escaping (Double?, Error?) -> Swift.Void) {
   
    let healthKitStore = HKHealthStore()
    let predicate = HKQuery.predicateForSamples(withStart: startDate, end: endDate, options: [])
    let sampleType = HKSampleType.quantityType(forIdentifier: type)!
    let sampleQuery = HKStatisticsQuery(quantityType: sampleType,
                                        quantitySamplePredicate: predicate,
                                        options: .cumulativeSum) { query, results, error in
                                         
      if results != nil {
        let quantity = results?.sumQuantity()
        let totalDistance = quantity?.doubleValue(for: unit)
        completion(totalDistance, error)
      } else {
        completion(nil, error)
        return
      }
    }
    healthKitStore.execute(sampleQuery)
  }