Cathe stated in a post that she originally thought to have STS be a 6-month rotation, but felt there were many people who would be unwilling or unable to commit to that length of time. She is the one who recommended doing each week twice before moving on to the next one if people were interested in extending the rotation past three months. She also suggested only doing the Meso 3 weeks one time through, rather than twice, which makes it short of a 6-month rotation, but most people are still calling it that.
As you probably experienced if you have started Meso 1, it moves very quickly. I found that repeating the weeks helped me get a better, smoother workout the second time through since I had notes from the week before and knew better what to expect.