You probably have something listening on port 80 for all IPs. eg:

$ netstat -an
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN
...

in this case ssh is listening on port 22 for all IPs. if you see 0.0.0.0:80 that probably means apache has taken port 80...