Also, there's a strong possibility that YouTube is automatically unsubscribing users from channels they've subscribed to if said channel hasn't been active (as in uploaded any new content) after X-amount of time - this could actually explain why subscriber numbers seem to keep fluxuating all the time, but people have also been mentioning they're finding themselves having to keep re-subscribing to channels they had already subscribed to.
Apparently, YouTube is becoming more and more adament about regular content - like I think they only consider a channel "active" if there's a new upload on a weekly basis. So that's really not fair to channels who can't maintain such activity, like mine.