"Microsoft recognizes licensing revenue as an OEM builds and ships [PCs] to a distribution point or to end customers," said Moorhead. In other words, machines that have been assembled and shipped, but not yet sold or deployed, would include licenses that Microsoft can legitimately count among the 40 million.
Without sales figures from Microsoft's OEM partners, it is impossible to tell how many PCs with already purchased licenses are not yet in customers' hands.
But the number could be significant. Last month, Microsoft said it had sold $783 million worth of Windows 8 licenses to OEMs prior to the OS's release, a figure up 40% over the same quarter prior to the launch of Windows 7. If that increase in licenses purchased has not been matched by a corresponding increase in PC sales -- and there's evidence that the latter have been disappointing -- Microsoft's "sold" total could be substantially higher than the number of machines actually in use.
Helm suspected as much. "It's possible that Microsoft has given OEMs incentives to order early and often," he said of Windows 8's high pre-order volume.
One thing is clear, however: The 40 million doesn't represent the number of PCs currently running Windows 8. "It's system sales as opposed to upgrade sales," said Moorhead of the difference.
So, too, for other licenses, which could be tied to PCs that, although purchased, are not running Windows 8 but instead have been downgraded to Windows 7, a common strategy by enterprises.
Still more of the 40 million can be attributed to the upgrade program Microsoft kicked off last summer that lets buyers of Windows 7 PCs acquire an upgrade to Windows 8 Pro for just $14.99. Microsoft recognizes that revenue, and thus moves the license to the "sold" column, when the upgrade is purchased.
Windows 8 had a slight advantage there over its precursor: Microsoft kicked off the Windows 8 upgrade offer June 2, 2012, three weeks earlier than the corresponding Windows 7 deal, which began June 25, 2009.
In the end, Reller's number may be relatively meaningless to Microsoft's core constituency.
"For enterprise customers, what matters is deployed Windows 8 units," Helm said. "That's what is going to determine the ramp-up of the new Windows 8 development platform, WinRT, which companies need to evaluate for use on tablets now and pretty much everything long term."
On that metric -- copies of Windows 8 being run -- Microsoft has always been moot. Others, such as Net Applications, have tried to step into the gap to estimate usage, but the numbers are always extrapolations.