Michael Arrington just made a mistake in claiming that Outlook 2007 is the most popular feed reader of subscribers of TechCrunch.
FeedBurner has very limited ways to measure subscribers and most of those are significantly flawed. On this case in particular I'll only pick on the Outlook 2007 issue.
I believe the Outlook number is probably 3 or 4 times smaller than reported by that graphic. So, instead of having 519,904 subscribers from Outlook 2007, TechCrunch probably has only 130,000 or so, making it the 4th most popular source of subscribers.
My case is based on how FeedBurner can compute unique readers. On the case of Outlook 2007 (or any email app), you are stuck between a rock and a hard place. Outlook 2007 doesn't send cookies. So, there is no way to do a similar unique count like Google Analytics does. The only number FeedBurner could use is the IP address.
Now think about your typical TechCrunch reader (no, they are not developers). They are your regular corporate Joe. And what Joe the Employee uses as a device to carry his work? A notebook! You know, those little computers you carry around, sometimes called a laptop.
Now, this laptop will know at least 2 IP address for sure. When it's at the office and when it's at home. But how about when you are having coffee at a Starbuck or Tully's. Well, that's another IP address. What about when you are at the airport, hotel or library... Another 3 IP addresses.
I have no data to back it up, but it's fairly reasonable to assume a laptop sees 3-4 or even more IP addresses per week.
This problem doesn't apply to Google Reader, BlogLines or any other online reader. It might not apply to other offline readers that support cookies as well, but who knows.
The bottom line is that you should never trust in a piece of data if you can't validate the assumptions behind it.