Exact traffic numbers might be not accurate, but traffic trend in the course of full year at that traffic volume is generally pretty accurate.

Those analytics bear very little resemblance to our actual traffic both in terms of volume and overall trend.

You are right, Quantcast shows different trend:

http://www.quantcast.com/github.com

Compete is often wildly off. Compare the different services that offer similar analytics and you'll see how often they disagree.