I use AudioQueue to record,why call the callback nearly at the same time

I am trying to use the AudioQueue to get the raw PCM audio data,but when the program is running ,it will first call four callbacks with an interval of 0.001s(seem to happen at the same time),and then after 0.5s it will call the next four callbacks repeatedly.Does anyone have come across this situation?

note:I make reference to an example (SpeakHere);

I set my buffer size as 2048 Bytes; here are the part of my programs:

_recordFormat.mSampleRate = 8000;

_recordFormat.mChannelsPerFrame = 1;

_recordFormat.mFormatID = kAudioFormatLinearPCM;

if (inFormatID == kAudioFormatLinearPCM){

      _recordFormat.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsPacked;

      _recordFormat.mBitsPerChannel = 16;    

      _recordFormat.mBytesPerPacket = _recordFormat.mBytesPerFrame = (_recordFormat.mBitsPerChannel / 8) *    

_recordFormat.mChannelsPerFrame;

     _recordFormat.mFramesPerPacket = 1;

}

while I am running the APP the print is shown as follow:(note the time)

2015-07-20 16:45:53.235 HelloWorld[4115:239431] bufferByteSize is :2048

2015-07-20 16:45:53.291 HelloWorld[4115:239431] we are turely begin recording

2015-07-20 16:45:53.802 HelloWorld[4115:239511] we are in callback

2015-07-20 16:45:53.803 HelloWorld[4115:239511] we are in callback

2015-07-20 16:45:53.803 HelloWorld[4115:239511] we are in callback

2015-07-20 16:45:53.803 HelloWorld[4115:239511] we are in callback

2015-07-20 16:45:54.313 HelloWorld[4115:239511] we are in callback

2015-07-20 16:45:54.313 HelloWorld[4115:239511] we are in callback

2015-07-20 16:45:54.314 HelloWorld[4115:239511] we are in callback

2015-07-20 16:45:54.314 HelloWorld[4115:239511] we are in callback

2015-07-20 16:45:54.824 HelloWorld[4115:239511] we are in callback