Oh wow, that is very interesting. I always thought it was Forbes cherry-picking content to repost, had no idea that it was Quora themselves selecting what to essentially "promote" on Forbes.
Also, now I know why @Quora retweets the Forbes versions so often! (It always aggravates me, personally - I usually click through to the Quora version so I can see other replies, votes, comments, etc.)
(Haven't checked, but assuming they're licensed under Creative Commons?)