NetworkStatsManager: querySummary returns the same data for different start time

Question

I'm trying to use NetworkStatsManager class to retrieve the data usage of my app. I want to track it once in an hour (intervals of an hour) and as I read queryDetails is not an option for me because it's just for 2-3 hours.

when I tried to use querySummary:

  private HashMap<String, Long> getNetworkStats(NetworkStatsManager networkStatsManager,
int connectivityManager,
@Nullable String subscriberId,
long startTime,
long endTime) {
  long foreground = 0;
  long background = 0;
  HashMap map = new HashMap<String, Long>();
  try {
    final NetworkStats networkStats =
        networkStatsManager.querySummary(connectivityManager, subscriberId, startTime, endTime);

    NetworkStats.Bucket bucket = new NetworkStats.Bucket();
    while (networkStats.hasNextBucket()) {
      networkStats.getNextBucket(bucket);

      if (bucket.getState() == NetworkStats.Bucket.STATE_FOREGROUND) {
        foreground += bucket.getRxBytes() + bucket.getTxBytes();
      } else {
        background += bucket.getRxBytes() + bucket.getTxBytes();
      }
    }
  } catch (RemoteException e) { }
  map.put("foreground", foreground);
  map.put("background", background);
  return map;
}

Call to getNetworkStats with two different startTime:

long endTime = System.currentTimeMillis();
long startTime1 = endTime - (3600 * 1000); //One hour ago.
long startTime2 = endTime - (3600 * 1000 * 2); //Two hours ago.
getNetworkStats(networkStatsManager, ConnectivityManager.TYPE_WIFI, startTime1, endTime)
getNetworkStats(networkStatsManager, ConnectivityManager.TYPE_WIFI, startTime2, endTime)

return the same data. Why?