Usually Pokemon games release with a full week in the US so if it did manage 1 mil there in 2 days then it's far from a decline. 5.5m always seemed like a bloated figure for first day sales physical only! Just a slight comparison with FIRST WEEK sales of D/P, B/W and B/W2 (without including others and Canada):
D/P: 3.2 million
B/W: 5.1 million
B/W 2: 2.5 million
Note first week sales of those are 2 days in Japan (same as X/Y), 2 days in Europe (Same as X/Y though Sundays don't count for much as mentioned though), and 7 days in the US (5 more days than X/Y). So really compared to B/W which was hyped beyond measure with a userbase of over 120 million, this is actually doing quite well.
One more note, even if it did around 2.5 million in Japan, then 1 million in US + 500k in Europe is actually pretty decent especially since the holidays are around and will most likely be kind to the game. Very good start imo.







