Vista Internet Use Up (and Mac Down) in October

Fortune’s Philip Elmer-DeWitt comments on the latest Internet market share numbers from Net Applications. Seems Vista got a bump in October while the Mac went down, and he’s curious as to why. 
It’s important to note that, as Philip himself explains, these numbers are an inexact measurement in the first place: 

The first thing to be said about these results is that Net Applications’ “market share” report doesn’t actually measure share of market as a percentage of revenue or unit sales. That’s the business Gartner and IDC are in.

Still, it’s interesting to look at the numbers to see what tentative conclusions could be drawn. 
The first thing I noticed when looking at Windows as a whole over the last year is that, while the trend is downward, there have been three points (Feb, Apr, and Jul) in addition to October where a gain was scored, so a monthly increase in and of itself is not out of the ordinary.

Further, Philip focuses more on Windows Vista, but even there the increase is not unusual. See the figures for Vista and XP for 2008 (rounded to the nearest half-percentge point):  

So Vista gained 7.5 points in nine months. The reason for this, I think, is obvious: Vista is getting sold on more and more PCs, so naturally its specific figures will show a steady rise. 
Meanwhile, however, XP shows a decrease at almost the same rate — 7 points — for those same nine months (other Windows losses came from older versions like Windows 98). This shows Vista’s licenses are coming primarily at the expense of XP and older Windows OSes, which is exactly what one should expect. 
As for the Mac, it shows similar traits as Windows. By that, I mean that it’s monthly numbers for the year show some losses along with the gains. Therefore, a drop for one month is not that unusual. The primary difference between the Mac and Windows charts is that the trend for Windows is down, while that for the Mac is up:

In my opinion, since October’s numbers don’t reveal anything particularly different than any other month’s, I think it’s fruitless to try to pick apart that one month. We’re just seeing the continuation of Vista gains (and XP losses) that we’ve been seeing all year. 
Still, if you feel there must be more to it than that, and need an alternate theory, I can postulate one as well as the next guy. For example, I could point out that late October was when Microsoft held their developers conference (PDC), with the subsequent Windows 7 blitz. That one event alone could have bumped Vista usage on the Internet as those users checked to see when their long national OS nightmare would be over.