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Fun fact; services such as YouTube have algorithms so complex, some other features can just be treated as a visual guide. This is exactly the case with subscriber numbers. Being subscribed to a channel does absolutely nothing, but make the arbitrary number grow bigger.

YouTube decides if a viewer is treated as a subscriber based on watch patterns and not based on that button. Thus, even if you're not subscribed to a channel, but watch it regularly, you're almost certainly treated as a subscriber. You've probably noticed that you're constantly being recommended the latest uploads of certain channels.