@Demonflavor The general trend looks accurate. We've done this comparison in RBGs to see if those maps have imbalances (they don't).- Holinka (@holinka) January 20, 2014
It's interesting to note that Holinka is quick to dispel the notion that maps are imbalanced. But let's look at the ones completely outside the RBG rotation for a moment. Isle of Conquest (IoC) is a really interesting one. It's one that it is definitely easier for Alliance to win, thanks to how the Docks is marginally closer to their base than to the Horde one, but in no way does it account for that magnitude of difference. Indeed, looking back at MMOC's 2011 figures tells a very different story.
What this tells us is which battlegrounds Horde downvote. Horde have a minor disadvantage in IoC, and so the experienced PvPers will downvote it. This means that the Horde players who are in IoC are often newer players, less likely to be able to stop the Alliance Docks-->Glaives-->Win strategy.
Alterac Valley is an even more interesting story. While there are many assertions that Alliance have a slight map advantage, it is a very slight one. Less, perhaps, than IoC. Honestly, it's as much rumor as it is anything else. And what's more, AV is the most downvoted battleground of all. So the Alliance's established strategy of "run to the end and hope somebody captures towers" will likely be easy for an inexperienced team to pull off against another inexperienced team. Basically, the Alliance has an established strategy. The Horde doesn't.
Looking at Strand of the Ancients, we have a battleground that even more clearly doesn't have any map bias at all. Simply because both sides are attacking and defending the exact same map. And yet the MMOC figures show a huge bias towards the Horde. Why is that? Well, here's a thought. Alliance, thanks to the huge win chance in Isle of Conquest, are less likely than Horde to downvote it. So, the two maps which Horde downvote, AV and IoC, leave them no additional downvotes. Yet, Strand is the third most downvoted map. So which map are Alliance players downvoting? Yep, Strand. And while that doesn't absolutely mean that all the experienced Alliance PvPers are downvoting it, a trend is easy to start.
I'm not sure just why Alliance downvotes Strand, given that it's one of my favorite battlegrounds. It may relate to more successful Horde teams thanks to a recognized strategy, but as a mostly Alliance player, we have a strategy too. Personally, I think that the issue is more that the game is very regularly lost or won by the end of the first round, so the second round is already decided. Because of this, if their faction has lost, many will just drop group after the round ends, meaning new players zone in to a lost battleground. This is bad for morale, and bad for Strand.
So while these numbers are considered fairly accurate by Blizzard, don't take them too much to heart. There's far more to them than "Horde are bad at AV and IoC and Alliance suck at everything else". It would be really interesting to see how Blizzard's internal numbers weigh up, but given the relative accuracy of these, it is odd to me that Horde outperform Alliance on several maps. And I'm not sure why. Maybe Horde has more PvP participation, maybe more PvPers roll Horde, but that seems odd given the strength of the Human racial, and how much it's complained about. That's a question we can't answer!