I’ve seen this misconception passed around a lot in our discussions, so I just want to clear up how a faction gets the 30% bonus. It is not based on your server, and it is not based on the shard you are on. If there are 2 full Alliance raids ravaging a zone and you still have the 30% buff it does not mean the Alliance are still outnumbered in that zone. It is based on the average participation in War Mode for each faction in the previous week.
From the Q&A today (summarized by WoWHead):
As a Horde player on a Horde dominant server, I will never see the 30% War Mode bonus.
Every weekly reset, the system looks at the relevant balance of the two factions in War Mode in the previous week and adjusts the percentages.
Faction Assaults have brought more people to World PvP and turning on War Mode.