Nmap reports host up when it isn’t

On an Ubuntu VMWare VM I ran:

sudo nmap -sP 192.168.0.*

This returned:

Starting Nmap 5.00 ( http://nmap.org ) at 2010-12-28 22:46 PST

Host 192.168.0.0 is up (0.00064s latency).
Host 192.168.0.1 is up (0.00078s latency).
Host 192.168.0.2 is up (0.00011s latency).
.
.
.
Host 192.168.0.254 is up (0.00068s latency).
Host 192.168.0.255 is up (0.00066s latency).

So, nmap reported that every ip in the subnet 192.168.0.* was live. The problem is I only have 4 live machines on 192.168.0.* so why did nmap report every ip address was live?

The ip address of the Ubuntu VM is 192.168.28.131
From this VM I can ping the live systems on my internal subnet 192.168.0.* and get the expected response. And if I ping a machine that doesn't exist I can get no response as expected.