Sigh.
PR companies don't make stupid mistakes like announcing 1 million when waiting another 6 hours would had allowed them a greater PR victory of 1.5 million.
From a marketing perspective, it's fairly easy to extrapolate the assumptions behind the 1 million announcement even though it's not a full 24 hours in every launch market.
When PR companies announce things in the millions, the subsequent metrics that matter are in the hundred thousands. My company, if we sold something in the millions, will only put PR value at 0.2 -> 0.5 -> 0.8 subsequently. Different companies do these differently, but rationally it's around the same ballpark, as PR is about targeting peak values.
If I were to guess, MS does not have more than 200k 'walk-in stock' for its launch. That, or they've extrapolated enough from what was sold for non-preorder quantities that the remaining potential sales are below 200k units.
If you don't physically have the quantities to push your PR messaging to 1.2 million, then there's really no point to hold back. Announcing 1 million+ is already the best message you can deliver at that point.