You're reading it correctly, it's just telling you the wrong thing.

51% of a very small self-selecting set of machines are down, mostly small businesses or home users which have obviously been shutdown or renamed.

If internettrafficreport automatically removed all devices which returned no results for 7 days, they'd be a slightly more useful resource.

As Ewantoo mentioned, ITR is pretty unreliable. We have found it works as a relative measure -- when their packet loss rises, we do indeed see consumer traffic fall, though usually not by the amount their graphs would indicate.