What does the S stand for when showing statisitics?

On the XBMC install in my living room, it shows an extra letter when pressing O, the letter S you see at the end, and it then shows a number of frames and a fluctuating percent error. Does anyone know what this means or how to fix it? The screenshot is from the PVR, but it happens on normal content to. Also, every once in a while the screen tears while playing, however on another XBMC install the error doesn't happen and the S statistic doesn't show up.

Any help would be awesome,
Thanks