iOS 9.1 AudioQueueAllocateBuffer requesting a fix buffer size, it returns a smaller one

In the new iOS 9.1 when sets the AudioSession requesting a fix buffer size, the OS returns a smaller buffer. Why does that happen?? In the early versions <9.1 it worked as a charm

// Create a new audio input queue
OSStatus result = AudioQueueNewInput(&mAudioFormat,
                                     IOSAudioRecorder::RecorderCallback,
                                     this,                  // userData
                                     nullptr,               // run loop
                                     kCFRunLoopCommonModes, // run loop mode
                                     0,                     // flags
                                     &mAudioQueue);
if (result != 0)
{
    Logger::Error(this, "Failed to create new audio input queue, result: ", result);
    mAudioQueue = nullptr;
}
else
{
    // Allocate memory for the buffers
    for (unsigned int i = 0; i < mNumBuffers; i++)
    {
        AudioQueueAllocateBuffer(mAudioQueue, mBufferFrames * sizeof(short), &mInputBuffer[i]);
        mOutputBuffer[i] = new short[mBufferFrames];
    }
}

And in the "RecorderCallback" I receive buffers smaller than the requested.

Any clue why does that happen?

The buffer size specified by this Audio Queue allocation call is a maximum of what can be returned. The iOS device hardware is free to return less data. You app has to accommodate this potentially smaller size, and/or may have to concatenate several returned buffers (in a lock-free circular FIFO for example) if you need a fixed length chunk of data.