With January on the way out, AdDuplex returns with an interesting look at Windows Phone usage for the month. Also worth discussing: Why Nokia Lumia sales were down quarter over quarter in Q4 2013, but actual Windows Phone sales to customers—and thus activations—were in fact up. It's all here in this month's peek at Windows Phone device usage.

As you may remember, AdDuplex bills itself as the largest cross-promotion network for Windows Phone and Windows 8 apps, empowering developers and publishers to promote their apps for free by helping each other. And each month it provides a tantalizing glimpse at which Windows Phone (and Windows) devices people are actually using.

This month, however, there are two things to examine. The Windows Phone device usage stats, of course. But also how we rectify the fact that Nokia's Lumia sales in Q4 2013 were down quarter-over-quarter (but up dramatically year-over-year, which is how we really measure such things), while Windows Phone sales to customers and activations were in fact up quarter-to-quarter in Q4.

Let's look at the AdDuplex usage data first. Some trends from this month's report include:

Lumia 520 is still number one. It's getting a bit boring since the Lumia 520 has pretty much been the Windows Phone story since mid-2013, but—shocker—it's still number one and growing, albeit more slowly, with 31 percent usage worldwide.

Low-end phones rule. When you combine all of the low-end Windows Phone handsets—the Lumia 520, 521, 620 and 625—you can see why Nokia moved more aggressively into this end of the market in 2013: They represent about 47 percent of all Windows Phones in use. And the number is much higher if you just look at Windows Phone 8 handsets, but AdDuplex doesn't provide those numbers for January.

High-end phones drool. The Windows Phone market seems mostly incapable of accommodating high-end handsets. Looking at the top ten worldwide, only one device—the Lumia 920—was ever marketed as a high-end device, and newer entries like the Lumia 1020 and 1520 don't make the cut anywhere. That said, here's one oddity: AdDuplex reports that there are currently more Lumia 1520s in use than there are Lumia 525s (the new low-end handset for China). But neither are anywhere near the top ten.

Nokia is still number one. Not much movement here either, but Nokia now accounts for 92.3 percent of all Windows Phone handsets in use.

Windows Phone 8 vs. 7.x. 78.3 percent of all Windows Phone handsets in use are running Windows Phone 8, vs. 21.7 percent for Windows Phone 7.x.

Windows Phone 8 Update 3 vs. GDR 2 vs. GDR 1. Looking at the three major updates to Windows Phone 8 that happened in the past year, AdDuplex sees that 67.3 percent of Windows Phone 8 handsets are running GDR2, which makes sense since that is the one that was most recently rolled out to most users. Update 3, which is available only to a limited set of device types on a limited range of carriers, has hit 18 percent usage. And GDR 1 is still kicking around on 14.7 percent of Windows Phone 8 handsets.

As for the supposed quandary over Windows Phone sales in Q4—someone on Twitter actually asked me if Nokia was somehow lying about its sales or had conspiratorially underreported Lumia sales in Q4 to assuage investor complains that it is selling its handset business to Microsoft—it's not all that hard to explain. And I was interested to see this week that AdDuplex's Alan Mendelevich agrees with my assessment.

It goes like this.

Back in Q3 2013, Nokia reported record Lumia sales, 8.8 million units, and since Nokia represents about 92 percent of all Windows Phone handset sales, it's safe to say that Windows Phone overall experienced record sales in the quarter too. But then in Q4 2013, Nokia reported that it sold 8.2 million Lumia handsets. That's a huge increase from the 4.4 million it sold in the year-ago quarter—the figure that actually matters—but it's also less than Q3 sales, which caused some handwringing in the less sophisticated parts of tech bloggerdom.

The thing is, the quarter over quarter drop-off isn't notable in any way. In fact, this isn't even the first time Lumia has experience a quarter over quarter drop-off. But since Q4 was the holiday quarter, the worry is that something terrible happened, and that Lumia sales fell through the floor.

Not so. Like most hardware makers, Nokia registers a sale when it one of its handsets is purchased ... by the channel. (Since, you know, that's when it gets paid.) So a big part of the record 8.8 million Lumias sold in Q3 was devices that were sitting in retailers and distributors, waiting to be sold to actual consumers. Those consumers purchased the phones in Q4 and in many cases delivered them to others as Christmas presents.

This theory was bolstered when Windows Phone's Joe Belfiore attempted to detune concerns about a supposed Windows Phone fall-off in Q4 in what is unfortunately a typically cryptic and incomplete way: He tweeted that "folks who think [that] holiday sales of [Windows Phone] declined are incorrect ... Activations more than doubled last holiday [quarter] and increased [over] each holiday month." And in his own blog post, AdDuplex's Alan Mendelevich provided the same theory I did above to explain how Nokia's sales numbers and Windows Phone activations are in fact perfectly consistent. And he has usage data to back that up.

In fact, Alan's post provides a few interesting bits of data: He estimates that there are now 38 million Windows Phone 8 handsets in use in the world, and 12 million Windows Phone 7.x handsets. These are actual handsets in use, not sales. That means that there were about 50 million Windows Phone users in the world at the end of 2013.

Not too bad.