I believe that site may have some quality links pointing back to it. Long gone were the days where you can fire up a link building tool and watch your site increase in its SERP ranking on a weekly basis, as it appears that the big G has finally addressed the churn and burn issue. Links from overspammed sites are now worthless, and those link list purchased are no longer effective. Furthermore, it's so difficult to even get a decent amount of good links from those list.
Well PBN would be one of the contributing factors. But I'm guessing there's also a possibility that 301 links from quality sites were used as well - sites that have TF>25-35 DA/PA>35-45.
