C# AxWindowsMediaPlayer fast forward

Question

I'm just starting out with programming and in one of the courses it gets me to make a basic windows audio player. It had no functionality for skipping forward or backwards so I thought I'd add that to the basic program. I added the required buttons (button3 for rewind and button6 for fastforward) and set their clicked code-behind as follows:

    private void button3_Click(object sender, EventArgs e)
    {
        MyMediaPlayer.Ctlcontrols.currentPosition -= 5; 
    }

    private void button6_Click(object sender, EventArgs e)
    {
        MyMediaPlayer.Ctlcontrols.currentPosition += 5;
    }

The strange thing (to me anyway) is that the rewind works perfectly, whereas the fastforward doesn't work at all. A) what am I doing wrong and B) why the difference between the -= and the += in this case.

Thx.

Jeff