It spills over to the US too since we get sharded with them. If you’re on the West Coast and play later at night, you’ll frequently face the enormous swarms of Alliance players from Frostmourne.
The “Alliance are totally outnumbered in war mode” thing is very much a regional EST/CST thing. You guys are just really loud in complaining about it, so Blizzard ends up giving free gear and massive bonuses to the Alliance players in other areas who massively outnumbered the Horde.