The > button shows the last image recorded to the card.. I think Jon os spot on.. the automatic image review is showing what is in the buffer before it is written to the card.
I have not noticed this on the 20D yest.. but with the MkII if I shoot a large burst of raw images, let's say 12,.. and then click the > palyback button.. It will show the 2nd or third image in the burst as that is the most recent one allready written to the card.. then as the images continue to write.. it will continue to scroll throught the remaining images as they are written to the card.. eventually showing my the last of the 12 frames.