I have no reason to doubt the success stories, based on what I know I've earned myself here over the years. I've never earned the thousand-plus a month that some of the "success-stories" are, but I don't write the kind of commercially-focused/search-focused stuff they do. Even with that, up until the end of this last summer, I was earning my few hundred a month on here. (I'm not earning that now. Then again, I've trimmed out more than half of the Hubs I had before, trimmed out most images because I didn't want them on my Hubs for my own reasons, won't promote my Hubs, and won't go back to "tweak"/update any of them, and don't write on here regularly.) The point is, I know that I've hit $500 a month with only a few hundred, mostly "non-commercial"/non-SEO-minded" Hubs, and have at other times averaged $300-$500) I don't know who has deleted what, but some of those "success-story people" had over a thousand Hubs at one time. If a person can pick up a few hundred dollars a month with only a few "commercial-minded/SEO-minded" type Hubs, why wouldn't someone be able to pick up several times that with several times the number of Hubs....
One look at the forums (or a lot of other places), and it's easy to see that earning and getting traffic with Hubs isn't as easy these days (at least for now, and who knows for how long..). Whoever is earning well now is generally writing the kind of Hubs that are required in order to earn well, but they're also most likely writing enough Hubs to have even lower earners add up.
Who/what any user says s/he is, and whether that's real or not, is a-whole-nother thing; but whether - when you can see the Hubs under a user's profile - those users (the "success stories") are earning what they say or not (with the particular profile/user account they say they are earning it).... I just don't have any reason to question. Also, keep in mind that some people are primarily "Ad Sense Only", while some use a combination of both Ad Sense and the Amazon affiliates program for earning. A whole lot of people will tell you that Amazon is where they earn their best income on here.
There's nothing at my profile now that would earn me now what I was getting before deleting some of my better stuff; but the real point is that if a person can earn five or six hundred dollars a month with Ad Sense only and only a few hundred Hubs... why would it be so hard to imagine earning that much more with Amazon and tons of Hubs???