The easier the zerg, the more favored the Alliance. I don’t think anyone can argue that to be honest …
Before the addition of reinforcements + timer in patch 2.3 ; there were many discussions on the forums about the issued AV had. Tom Chilton agreed with these issues stating that Alliance was winning 70-80% of the Alterac Valley. That statistics was for TBC but the battleground had not been changed since patch 1.10 so you can assume it was the same in Vanilla.