Best answer by Mathis
Our followers service have been undergoing a major maintenance to see if we can provide updates in faster intervals. We're hoping this will soon improve our users' experience with the followers service. However, please note that it is rather normal for your number of followers to change both ways, depending on how many users start following you (for which you're being notified), against the number of users who decide to no longer follow up (for which you're not notified).
All the best