Ignoring loading time & customs, the travel time from China to US West coast by cargo ship is 11 days, and then you have shipping time to and from some port.  If it's final destination is the East cost then it will take a lot longer.  A plane can fly the same to West or East coast in less than 1 day.

More so, from China to the UK it takes longer by cargo ship, more like 3 to 4 weeks.

They could have used the strategy of shipping more by ship to the US, and shipping to Europe by air.  Especially, China to UK is much closer by air than China to USA.  Either way, introducing air shipping will have moved some supply from January into December.

Don't take my word for it though, this is just a possibility to consider.

On the history of the Playstation , Kaz talks about how during the PS2's NA launch they decided to airlift a couple of weeks of extra stock to help with high  demand , it really is something they only consider to do on launch as it just gives them a little more breathing room until a full shipment arrives , also they have to absorb the extra cost .

 To it's impact on the next shipment it depends on where they got the stock from , iirc the ps2 stock came straight from the factory in Japan and wasn't  culled from a warehouse awaiting shipping as they had shipments onroute..

If you look at most past systems, you see that their debut quarter and the first quarter of the following year compare very favorably to each other, so 8+ million is a distinct possibility for the PS4. Here's VGC's estimates for total global sales for all three seventh-gen systems during their launch quarters:

Wii: 2954k
PS3: 1252k
360: 1178k

Here's how they did the following quarters:

Wii: 2883k
PS3: 1606k (incl. Europe debut)
360: 1246k

Here's how NPD shows for sales just in the U.S. for the three and and the PS2 in their launch quarters:

PS2: 1087k
Wii: 1080k
PS3: 688k
360: 607k

And in the following quarter:

PS2: 1007k
Wii: 1030k
PS3: 501k
360: 603k

As we see, these systems' first two quarters were very close to each other. However, this doesn't always happen. The counterexamples are the Xbox, GameCube, and Wii U, which had pretty big launches but they dropped pretty sharply in the following quarter, at least in the U.S. According to the NPD, the Xbox dropped 71.6% from launch quarter to Q1 of 2002, while the GC dropped 79.7%. The Wii U dropped 78.5% from launch quarter to Q1 2013. However, they were clear underdogs in their respective races. In the sixth generation demand for the PS2 eclipsed that for the Xbox and GC, and demand for the Wii U obviously hasn't been too great. However, there is clearly strong demand for the PS4 and XBO, and they have all the ingredients to make for successful systems, which makes me think they'll behave more similar to the PS2, Wii, 360, and PS3 in the short term. Of course, besides demand there are other factors to consider, including supply as well as whether their launch holiday sales were inflated due to a growing "preorder culture" combined with the longest wait between generations ever, which caused a lot of people to line up on Day One. Still, I think that globally the PS4 will likely breach 3 million and the XBO should breach 2 million, thus bringing their LTD totals to around 7 million and 5 million respectively. Of course, nothing is guaranteed.

It doesn't matter where they culled from if supply is limited by manufacturing capacity.  Express airlifting some units from the factory will just allow them to deliver those units sooner, instead of moving via the cargo ship route.

I.e. it doesn't increase overall production, it just leapfrogs some shipments so they arrive sooner rather than later.  In this case we just have more units arriving in December instead of January.

Now if Sony were somehow able to ramp up production to 2 million/month then no problem, but I think that's unlikely.  They had forecasted 5 million by end of March before.  It wouldn't make sense to prepare manufacturing for around double the amount they forecasted.