When I look at site logs I see large numbers of entries from robots claiming to be multiple versions of IE on multiple versions of Windows asking for pages at the rate of several per second all from the same IP, and not asking for stylesheets or images.
I believe the reason for this is the robot owners suspect the robots will be discriminated against if they don't pretend to be IE, but they don't want to be identified by the combination of IE and OS version. However, I'm convinced there is a lot of traffic out there which claims to be from Windows but is nothing of the sort, inflating MS's apparent market share. This makes any attempt to assess market share from website hits too unreliable to be useful.