Those are the nodes which allows incoming connection. It's methodology gives an inaccurate sampling of the actual node count as it doesn't take into account the nodes that doesn't allow incoming connection.
I knew I would get a comment like this.
Do we really think there are ten of thousands of nodes not accepting connecting being spun up in secret? Or can we go with the simpler explanation of not many people are bothering to spin up a node and we shouldn't be making it harder to do so because at some point it may or may not be a problem.