StumbleUpon sending more traffic than Google?

I'm a bit intrigued with the analytics stats being recorded on a site I'm working on at the moment and it's not just Google Analytics recording the same thing, but AW stats too.

What I'm finding most odd is that for this particular site, StumbleUpon.com is referring more traffic than Google. According to stats, in the last month or so, StumbleUpon has referred about 34% of visits and Google about 27%. In most client's log files, Google is way ahead of anything else. The other thing I find odd is that although the site has been bookmarked on StumbleUpon (not by me I might add!), there's hardly enough traffic from this one link that will send more traffic than Google.

You can see what I mean in this image:



I'd be interested to hear if anyone else has had any strange things being recorded by their log file stats, or if you know why this might be happening!