The counts that Alexa and Quantcast provide are their best calcluated estimates. When I was tracking out site traffic at FamilyLink/WVR we determined that Alexa and Quantacast were typically 10-20% lower that actuals. We even found that Omniture was also typically low for some reasons even beyond human error. We experienced this with both Google Analytics and Omniture.
However, since these traffic counts are probably similarly lower than actual for all sites, it is still a good measure of traffic. All of is still valuable as a traffic tracking tools that are help many of us.