nmap on guest system

[Go6a]

Host: windows 7 64, guest: ubuntu 12.04 server. I setup bridge between host and guest, host has wifi card with ip 192.168.1.4 and guest got automatic ip 192.168.1.10. Also I have router in my network, with ip 192.168.1.1. If I run

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nmap -sP 192.168.1.0/24

on guest I got

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Nmap scan report for 192.18.1.1
Host is up
Nmap scan report for 192.18.1.10
Host is up

But

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 ping 192.168.1.4

works fine.
Why my host is not visible by nmap but visible by ping?