This also seems to happen with my subscribers, I am not sure if it's based on the updates or not, but it feels weird when I see a user who subbed before suddenly appearing at the top of my sub list like he unsubbed and then subbed again. If it's really because of the stories updating, that would be very bad for me since I update around 4 times a week for 3 different series. XD
Then again I'm happy they're subbing again. :)) It takes dedication from their part, which I'm very appreciative of.