I believe it has to do with how things are ordered. Store will order cases of toys based on when they need stock. US toys operate in seasonal waves with some variated case assortments.
So instead of the 4" Gold Ranger "releasing" on a certain date, it gets packed in a case with other new figures for that season.
As far as I know, a lot of Japan's ordering works on a weekly basis (at least this is how CS Toys works), so Japan has a "this toy releases this week" structure, instead of the Seasonal structure the US does.