I suspect it’s because they contracted for two more seasons and then released each of those seasons over two consecutive years with enough episodes to fill two seasons each.
E.g. Season 6 had episodes released in June 2010 and July 2011. It’s a pretty reasonable expectation that that would be two different seasons.
For torrenting and Plex purposes the TVDB is the ultimate source of truth, If they said it was 14 seasons we’d be using 14 seasons for those numbers because it makes our automation work.