I bet that there is negative correlation between NoScript & StumbleUpon.

From StumbleUpon's advertiser FAQ:

A large portion of our Firefox users have added the NoScript add-on to their browser. This is one of the top-10 most popular extensions for Firefox. This extension blocks any javascript calls that the user doesn't approve of. Blocking javascript causes Urchin, Google Analytics, Webtrends, etc to not work. These tracking services never see the traffic because NoScipt blocks it.

You may see a larger difference in reported traffic than with other advertising services because StumbleUpon has a much higher proportion of Firefox users -- and in particular, a much larger proportion of people who use browser extensions -- than a typical audience. In essence, StumbleUpon's early-adopter user-base is much more privacy and security conscious than the typical internet user, and they block tracking services as a result.