Video is encoded to a frame rate, but I believe trickplay always needs to land on a "key" frame, which is the first frame in a sequence of frames that whatever compression is being used determines is most important to encode with as little loss of quality as possible.
Does a DVR track frame rates or does it treat it simply as data? [...]
That's why it sometimes appears after stopping FFX1, which has no autocorrection applied to it, that the video still jumps backwards before resuming play. I believe that happens when the closet key frame to the stopping point is before the resume point, and the HR needs to "back-up" to find it. Otherwise, if it always jumped to the next keyframe, there would be a risk of a dropped word of dialog.
MPEG-2 by it's nature is encoded with more key frames than MPEG-4, which is why MPEG-2 file sizes are larger. As a result, tho, MPEG-2 trickplay is smoother.
I think trickplay is an area where the HR's were being unfairly compared to the DirecTiVo, because the HR10 only had to deal with MPEG-2 files. Comparing MPEG-2 to MPEG-4 trickplay is really comparing apples to oranges.