readfile() and GetOverlappedResult() return lpNumberOfBytesTransferred lenght problem.

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VisualC++

I have a question

I declared a USB HID DEVICE, HID report as follows:

Usage Page (Vendor-Defined 1) 06 00 FF
Usage (Vendor-Defined 2) 09 02
Collection (Application) A1 01
Logical Minimum (0) 15 00
Logical Maximum (255) 26 FF 00
Report Size (8) 75 08
Report ID (1) 85 01
Report Count (7) 95 07
Usage (Vendor-Defined 2) 09 02
Input (Data,Var,Abs,NWrp,Lin,Pref,NNul,Bit) 81 02
Usage (Vendor-Defined 2) 09 02
Output (Data,Var,Abs,NWrp,Lin,Pref,NNul,NVol,Bit) 91 02
Report ID (2) 85 02
Report Count (63) 95 3F
Usage (Vendor-Defined 2) 09 02
Input (Data,Var,Abs,NWrp,Lin,Pref,NNul,Bit) 81 02
Usage (Vendor-Defined 2) 09 02
Output (Data,Var,Abs,NWrp,Lin,Pref,NNul,NVol,Bit) 91 02
End Collection C0
USB HID device success send 8 bytes data to pc. (01 35 00 00 00 00 00 00)

HidP_GetCaps(pPreData, &cap) get cap.InputReportByteLength = 64
Part of the source code:

{
memset(ReadReportBuffer, 0, sizeof(ReadReportBuffer));
ReadFile(m_hReadHandle,
ReadReportBuffer,
64, 
NULL,
&ReadOverlapped);
}
dObject = WaitForMultipleObjects(2, hArray, FALSE, INFINITE);
if (dObject == WAIT_OBJECT_0)
{
if (!MyDevFound)//Device extract also set the event 
{
continue;
}
GetOverlappedResult(m_hReadHandle,
&ReadOverlapped,
&nBytesRead,
TRUE);//you can also set the last parameter False
if (nBytesRead != 0)
{
m_strTemp = ReadReportBuffer;
m_strLog.Format(_T("Read the Report Data Length is %d.(%02d:%02d:%02d)"),
nBytesRead, sysTime.wHour, sysTime.wMinute, sysTime.wSecond);
m_strLog += m_strTemp;
Display_Info::SendEvent(m_strLog);
//Display Receive Data
pDlg->DisplayDataHex(ReadReportBuffer, nBytesRead);
}

why nBytesRead value = 64?