The logs make Rogues of all specs and Fire/Arcane look worse, and Unholy better, than they actually are because it aggregates their scores across all fights. Unholy is the spec of choice for every fight in the instance, while Combat/Sub and Fire/Arcane is largely a choice that depends on the individual fight, and thus leads to lower aggregate scores. If you look at the statistics for say, Gruul, instead, you get the following:
https://www.warcraftlogs.com/statistics/7#boss=1691
Clearly, Sub performs very well at the moment, beaten only by two hunter and mage specs that I think we can all agree are long overdue for a sizeable nerf. Similar results are seen on fights with heavy cleave for Combat, but because the choice between the two specs is so binary based on fight (i.e you never, ever, EVER play Combat on Blackhand unless you simply don't care any more), the aggregate scores across all fights for the Rogue specs are weaker than the aggregate score for Unholy. And that, kids, is why aggregate scores are a trap statistic.