I've had 23 reviews over 9 versions of my add-on. Some of these will be individuals reviewing my add-on multiple times. I haven't bothered to count, but I guess if you counted each person who has reviewed then it would add up to 15.
This is the correct explanation. Only the latest rating counts, but the old reviews are kept. Version 1 might've been junk, but version 2 may be great, so we try to handle that by only counting your most recent rating, while keeping your old review text around.