Well, there are also many pledges that are sort of "forgotten" (for lack of a better word), especially from people who pledge to numerous creators.
You kind of have your pledge running month to month and it just stays there, especially if said pledge is trivial compared to your total sum. That's an occurrence on many creators' accounts, and way more evident to some of those creators.
Other cases might involve just lowering your pledge amount instead of unpledging completely. Someone might've pledged for, say, 10$ when BB was still active but dropped it to 1$ just to get the news. He's still listed in the total pledgers amount, but the payout is surely changed.