According to Reddit's own Google Analytics data - which is arguably far more accurate than any third-party measurements - Reddit has over 8 million monthly unique visitors, 36 million visits and over 400 million monthly page views.
Why the discrepancy? Is it a technical issue? Reddit admins themselves don't know. In a blog post, they're asking for help: "Can you grep the server logs to see how many visitors we send your way, and how it compares to other major referers? That kind of information would be invaluable."
From our experience, web traffic analysis services are much better for determining trends than comparing absolute numbers. Many of them, such as Compete, are U.S.-biased; some may be biased with regards to what operating system or browser the visitors are using. Sometimes, their numbers fluctuate wildly: recently, several sites wrote about Digg losing a third of its traffic in just one month, according to Compete. Looking at the same graph now, Digg's traffic numbers look vastly different.