Yeah, StumbleUpon do that with their links, it confused me too at first.

Instead try using http://www.stumbleupon.com/url/www.yourwebsite.com/

You'll be able to see the review page that people are stumbling from. One one of my websites receives average 200 unique free stumbles per day and I have since become a paid advertiser, though their categories for targeting are very limited, imho.