The subscriber count in the Smart Push Marketing app is accurate. When someone accepts notifications from your website, they will automatically continue to receive your notifications as long as they haven't cleared their browser's cache and cookie settings and their browser settings are not configured in a way that prompts them to accept the notification each time.
The majority of your website visitors will not clear their cache and cookies often and will not use the option that makes the browser ask them for permission to receive notifications every time even if they've previously accepted.
When the cache and cookies are cleared from a browser, they will have to accept notifications again and will be counted as a new subscriber.
So while it's possible that the same person will be counted more than once as a new subscriber, it's unlikely to happen often. Every time the App sends web push notifications for a store, it checks if the subscribers are still active. If a subscriber is no longer active, the App will remove him from the list automatically.
If you believe a visitor has been counted multiple times, you can click on the Subscriber & Segment link and tell us which IDs to remove.