We see the same for NewsCred... our traffic is an order of magnitude larger than what Compete reports. We're thinking about getting Quantified for that reason, but I'm not so sure about sharing every little detail of traffic information with the whole world.
You can hide data fairly selectively on Quantcast, FYI. (As of one of our more recent releases people can ask you for access to your hidden data, which is nice if you sell ads.)
Full disclosure: I work for 'em so obviously I'd encourage you to go for it. :)
I have two sites that get 5-8 times the traffic shown by compete's graphs. Because of this, I just never take compete.com's stats as reliable at all.