WriteFileEx completion routine succeeds, but bytes transferred is incorrect

I'm communicating between two processes on different machines via a pipe, using IO completion routines.

Occasionally, when the completion routine for WriteFileEx gets called, the completion routine parameter dwErrorCode is 0 (i.e. no error), GetOverlappedResult returns true (i.e. no error), but dwNumberOfBytesTransfered does not match nNumberOfBytesToWrite in the call to WriteFileEx. I only see this on the client end of the pipe however.

If the number of bytes transferred does not match the number of bytes that was requested to transfer, how can this be deemed a success?

This is how the client's handle to the pipe is created:

mHPipe = CreateFile(pipeName,                 // pipe name 
                    GENERIC_READ |            // read and write access 
                    GENERIC_WRITE, 
                    0,                        // no sharing 
                    NULL,                     // default security attributes
                    OPEN_EXISTING,            // opens existing pipe 
                    FILE_FLAG_OVERLAPPED |    // overlapped
                    FILE_FLAG_WRITE_THROUGH,  // write through mode
                    NULL);                    // no template file 

// do some checking...

// The pipe connected; change to message-read mode. 
DWORD dwMode = PIPE_READMODE_MESSAGE; 
BOOL fSuccess = SetNamedPipeHandleState(mHPipe,   // pipe handle 
                                        &dwMode,  // new pipe mode 
                                        NULL,     // don't set maximum bytes 
                                        NULL);    // don't set maximum time 

Can anyone see why this would happen?

Thanks

EDIT:

The relevant WriteFileEx code is as follows:

void WINAPI CompletedWriteRoutine(DWORD dwErrorCode, DWORD dwNumberOfBytesTransfered, LPOVERLAPPED lpOverLap)
{
  BOOL fWrite = FALSE;
  LPPIPEINST lpPipeInst = (LPPIPEINS