The official tweet button for Twitter is in partnership with Tweetmeme who are one of the biggest providers of the "Retweet" button for websites all across the world. Tweetmeme also accounts for 750 million daily retweet button impressions. It definitely made more sense for Twitter to use the technology behind Tweetmeme, since it also accounted for shortened URLs.
In future, all Tweetmeme buttons will redirect to Twitter’s "Retweet" button by default and the Tweetmeme team will continue to work with Twitter on the Retweet button and other projects.
The Retweet button from Twitter is easy to integrate and makes use of Twitter’s own URL shorterner t.co, which it also uses to provide a brilliant spam protection mechanism. If you want to make use of the official Twitter Retweet button, head over to this link and create your own. Watch a video of the official Retweet button in action below.