Alliance actually win AV fairly frequently on most servers. There's a good reason, too:
the alliance simply have the numbers to keep up with the rediculously long game. As it drags on, the horde's numbers usually dwindle. If not, the crappy alliance players who soon become bored with the BG and quit are replaced with competant players who know what they're doing.
That's how it is on Feathermoon, anyway.