A subscriber represents a listener, someone who has gone to your campaign, linked their Spotify account and then stated they want to save your album to their library or add it to their playlist.
Your "Subscriptions" count, will update when we subscribe your user to your release, add it to their playlist and have them follow your artist profile, all of these count as individual subscriptions. So let's assume a user chooses to add your release to their library, add it to their playlist and follow you, this would count as 3 subscriptions.
If your Subscribers and Subscriptions counts differ a lot, don't worry, this is perfectly normal! 😄