I suspect it's because you're playing the file through the PVR addon, so tvheadend is effectively streaming the file to XBMC rather than XBMC knowing anything about the file itself. If you map a path to where tvheadend stores the recordings and play using Files mode, you may have better luck.
I'll check on my system for you when the kids leave it alone :-)
Oh, and on a normal XBMC file, you can always jump to a specific time by typing a timecode on your remote (HHMMSS format - so jumping to those last ten minutes is 015000).