Maybe they're shifting their shipments to the areas with the highest demand? Since EU wasn't that sweet on amiibo they might've allocated more to the US to make their situation slightly less dire. Just a guess.
Well my thought as to why the US gets our Wave 4 and Wave 5 at the same time in May, get half of our Wave 6 in July, a month later than us and get the rest of Wave 6 and Wave 7 in September, two months after us, was for them to build up enough stock for there.
All it has done, though, is convince the more hardcore amiibo fans to import from Europe or Japan to get them when we do.