Well a lot has to do with how the ISP or the network that the person is behind. in a workplace network the public IP for all computers behind the firewall is the same so therefore multiple people could be shown to have the same address. This could be quite large number of people depending if it is just a business or a WAN connecting multiple sites over large areas. Just like those who have mutliple PC's behind their Firewall.

Agoracom's tool is suposing that everyone is connecting from home and their modem's address is publicly viewable which may not be the case.