If someone gets some facts wrong and unwittingly repeat them, then that's unfortunate, but doesn't mark them as a liar or stupid or bad person. But if they keep telling the wrong facts, and keep going to the same sources of information that give them wrong facts, then that shows that they just don't care about the facts or the truth. A comparison to conspiracy theorists is a good one. If he made up stories in his biography I find that interesting, but not a reason to vote for or against him. I know a lot of memoirs and biographies have exaggerations to make stories work better, or just have honest mis-remembering things. It wouldn't surprise me if he made up stuff. I would like to see the stuff from his book to be investigated, just because it would be interesting to see how much he might have made up. But I hope he's brought down by his constant disregard for actual facts regarding science and politics in the current day, not for possible lies about his childhood.
I am much more concerned with Carson's present day wackadoodle beliefs than his lies. For the life of me, I cannot understand how Carson successfully navigated medical school with YEC views that are incongruent with how the world, including biology, actually works, unless he came to his current beliefs sometime after achieving his degrees. The pyramids story is a real doozy, because even if he really believes it, he should be intelligent and circumspect enough to know to keep it under his hat, but he is not. Not only does this show a person who is not very smart or self-aware, but someone who believes what he is told without question if it comports with dogma he has chosen to embrace.
I think that prior to his run for president Carson basically spoke in terms of parables. Ever since Jesus, (and probably before) preachers have peppered their sermons with ancedotes
http://www.snopes.com/glurge/glurge.asp that clearly illustrate whatever point they are trying to make. For this purpose it doesn't really matter whether Carson actually was the only honest person in his class, or a dying child actually saw the angel of his dead grandfather cure his cancer, or a Samaritan actually stopped to rescue an injured Jew. All that matters is that it feels true and supports his view point. Further saying the story happened to you rather than to someone else just make it all the more personal and inspiring. It makes a much better story to say that it was your niece who was cured, instead of just some child you heard of, even if you don't actually have a niece. So long as it inspires the faith of the listeners its all good, and the actual facts are irrelevant.
If the audience is supportive of your view and wants to have their faith inspired then this strategy is just fine. No one will bother checking it out, preferring to take it as true on faith. In fact, if pressed some in the audience will admit that what was said may not have actually happened but still think that the point of the story was right. The problem is that now that he is on the national stage and no longer in front of an entirely supportive audience who are more interested in facts than in faith, this strategy will blow up in his face.