The subscriptions API shows only current channel subscribers (including those who cancelled but are still active until their subscription expires). There is no way to retrieve a list of cancelled subscriptions.
The created_at field is when a current subscription started. By current subscription, I mean without cancellation. If you cancel and resubscribe manually, that starts a new subscription and the created_at changes. Twitch’s twitchnotify in chat adds up the months from every subscription to a channel, including past subscriptions not shown in the API.
If you’re only checking subscription state on a per-user basis, you might want to just use the /channels/:channel/subscriptions/:user endpoint so you don’t need to pull the entire list of subscribers.