First of all, m_throughput's type is uint32. And, if m_lastThroughput is greater than m_throughput, the result of '((m_throughput - m_lastThroughput) / m_lastThroughput)' should be negative value like following algorithm. 

if (((m_throughput - m_lastThroughput) / m_lastThroughput) > m_throughputRatio)
{
   m_beta = m_defaultBackoff;
}
else
{
   m_beta = m_minRtt.GetDouble () / m_maxRtt.GetDouble ();
}

But! since m_throughput's type is uint32, overflow is occurred. So, m_beta's value will be mostly m_defaultBackoff. 

Finally we suggest change algorithm like that

if((m_throughput - m_lastThroughput) > 0) { 
    if (((m_throughput - m_lastThroughput) / m_lastThroughput) > m_throughputRatio)
    {
       m_beta = m_defaultBackoff;
    }
}
else
{
    m_beta = m_minRtt.GetDouble () / m_maxRtt.GetDouble ();
}