Every time I've heard someone talking about the Horde:Alliance ratio on their server, it's always based on uselessand irrelevant data, like how many people were involved in that raid on Crossroads, or how many times X player got ganked in Alterac last night...
Likewise, there's always ******** about how the other side PvPs. "Those Alliance never come out and fight unless they have a 4:1 advantage," quotes my girlfriend now and then.
It's all perception. If you want hard facts, find the quest that lets you get data from the Census guy, and get your facts there.