I believe the official Blizzard stats confirm Horde dominance in most BGs aside from IoC and AV. The IoC layout clearly favours alliance because of the position of the docks and the massing effect glaives have on the game. AV also favours alliance because of the bridge.
The other battlegrounds are all very symmetrical though, so it makes little sense that one side should have an inherent advantage. And since I don't believe the OP in his assertion that Horde are simply buffed 10%, there must be some other explanation, and the only logical explanation is that Horde are better at pvp.
Which seems strange, but I have a plausible explanation:
It's about subscriber numbers: Alliance outnumber Horde, ie more people play Alliance than Horde. One immediate implication of this is that if you play Horde, you get to play more battlegrounds. Why do I say this? Well imagine there are 100 people queuing for Horde and 200 queuing for Alliance in one evening. Now lets say that 40 Arathi Basin battles happen that evening. That means that there will be 600 Arathi Basin spots available for each faction. Because Alliance spots are divided by 200 people, each Alliance member gets 3 games. Because Horde spots are divided by 100 people, each Horde member gets 6 games.
This means Horde players get more experience and more opportunity to get better at the battleground. Not only that, but Horde players get more Honor points in an evening, meaning they land up with a gear advantage which then sets up a feedback loop - as they win more Honor, the gear gap grows, they win more, get even more honor.