If you think about it like a computer network, most nodes have fewer neighbours than their neighbour nodes. Most nodes are endpoints with only a few neighbours, but some nodes are networking midpoints with a lot of neighbours, and every endpoint knows at least one midpoint.

On a network segment with one gateway and ten endpoints, all nodes can see ten other nodes... except the gateway, which can see more nodes outside the segment. The average is therefore higher than ten, so most nodes see less than the average.

Hrm, I'm not sure if that clears it up at all. Maybe :)