If the ratio on specific realms is bad, the problem would be exacerbated. When it’s 70:30, then the minority faction on the server dictates the total amount of matches. Assuming 100% of the server is online and queuing, then 40% of the majority faction is left in a queue.
However, if the total cross realm ratio is more balanced than individual realms, then a lower % of the majority faction will be in the queue.
While many servers seem Horde populated, there are also seemingly severely Ally imba servers, such as Heartseeker. This will help mitigate the terrible faction balance on Stalagg for instance. Moreover, many PvE realms are supposedly heavily Ally skewed. If these realms are all within the same battlegroup, then the ratio becomes even more balanced.
This assumes that we are seeing severe imba across all realms at all times, but different realms have different imba at different times. I expect the queue times will vary.