I want to quote my favourite answer on this subject.
This was said by Trulylucky on another thread regarding this subject and I think it applies here perfectly:
"I have to say, you certainly outdone yourself with that post.
Let me ask you, because you are obviously in an extreme state of zugzug.
If horde is as powerful and strong in PvP as you claim, how come Alterac Valley is the only battleground where horde is able to assume complete domination?
Why is that horde struggles when against equal odds, such as it is in other battlegrounds, but when in Alterac Valley where they are at a supposed map disadvantage (according to you), they somehow shine through and win 95% of the games?"
It is from this thread: