That final interview was probably just to refresh their memories about you, and to look for any flaws they missed the first time through. Anyone can look good on paper, most people can look good in person for a short period of time, but very few people can keep up a charade for 3 consecutive interviews. They are just looking for the best possible fit based on the remaining candidates. It's probably more about how your personality will mesh with the company than anything else.
I interview a lot. After you've interviewed 50+ people, they all start to look and sound the same. Even so, the best candidates are pretty easy to spot.