The Microsoft Kinect has been explosively popular, certainly to the tune of 2.5 million sold outright in the first month of its debut, and that was before the shopping holidays rolled around. I am emphasizing the word “sold” for a reason here, and that’s because last night Ballmer provided some figures on Kinect sales that made a twist on the concept of what makes a sale—he used a figure for sales which represents the total number shipped to stores, not the total number sold in retail. Although it is unlikely that there’s really that much difference, it was at least polite of Microsoft to explain the difference.

All Things Digital discussed this with a spokesman and explains:

Tonight, Ballmer provided an update on just how well it’s selling. In its first 60 days on the market, he said, eight million Kinect sensors were sold, outpacing the five million units the company had forecasted in 2010.

But those figures require a little more explanation than was given:

The 2.5 million sold in 25 days counted the number of devices consumers purchased, whereas the eight million figure reported today represents the number of devices shipped to retailers.

The discrepancy between those two is likely fairly slim. Up until recently, inventories have been really low, and retailers have been sold out in some circumstances. For instance, the Xbox 360 with 250GB, bundled with the Kinect, remains sold out at Walmart.com, but is available at Amazon.com.

The wording may be a little bit of finagling with English, but the reason is that the original figures had been carefully data mined from the retail industry, a methodology which was labor intensive and extremely expensive. Representatives at Microsoft had to call all of the various outlets and get their estimates at how many had sold from their shipments—something we now understand may have been unnecessary at the time as the units were literally flying off the shelves. That method gave a very precise “units sold” number which generated the 2.5 million.

This round, Microsoft is gauging instead by how many have been taken by retail outlets in shipments out, and calling that sales. Of course, from their statistics, Kinect inventory at retail outlets has been extremely low and they often sell out, so chances are the ship rate is not very far off the sales rate.

This is marketing, after all, and being in the ballpark in this estimate should be good enough.

The article also included a couple other Xbox sales milestones I’d love to share with everyone:

• 8 million: Number of Kinects sold in first 60 days;
• 50 million: Xbox 360s sold to date;
• 30 million: Number of Xbox LIVE members.

With the Xbox 360 being such a powerhouse and Kinect being so popular, it’s likely they’re feeding off each others sales and though the holiday crush has finally ended, it’s unlikely the popular momentum of the device will wane that much.