The reason you see some of the same people reranking is because they are achievement hunters. A lot of my friends are doing top 12 ranking for a third time not because of any interest in having a 3rd copy of the title or bragging rights, but because SE made the achievements themselves separate. Therefore they must rank to get the achievement points. Many of them were actually annoyed on learning this.

If SE had not made it so that the achievements and their points were unique for each phase, I suspect the boards for top twelve would be a lot more open.