Could it be that the Daily Kos has lots of people who designate it as their homepage so that whenever they open their browser the homepage loads and gets counted? That would explain both the high visitor count and the low average visit durations.
It could also be attributed to the usage of multiple tab browsers, like NetCaptor, for instance. Every time you open the browser, the same sites you visited pop up again if you set the options up that way. I have several sites that I keep in my browser for quick and easy access.