Quantcast provides Quantified sites with audience data based on directly measured census-level traffic. As a result, Quantcast audience data should align with a site’s internal reporting. Syndicated services typically report data based on panels, projecting a publishers’ total traffic off a small subset of their actual traffic.
The syndicated services may also be reporting different aggregations of domains, subdomains or even specific pages. As a result, traffic assignments may not line up between services. In such an environment, resulting data is not an exact apples-to-apples comparison.
Another possible difference – Quantcast audience data is based on visits, which may not be the case for other services that report traffic on basic audience composition.