How can the number of peers from the same IP address be counted?
That monitoring page states that it counts hosts from 'addr' messages seen on the bitcoin network. 

An 'addr' message contains the IP number &amp; port number of a bitcoin client. Since all bitcoin clients use the same listening port number the IP number is the only information to count by. 

When a bitcoin client connects to a peer it does send out an extra number to identify it by. However there is no friendly way to force all clients behind an IP number to connect to you so you cannot capture that number for all those clients.