High udp port are open temporarily

[pch0317 <pch0317@gmail.com>]

Hi :)

I install new server with Debian stable (base system only with ssh and 
bind).
I scan port with:
nmap -sS -sU -T4 -A -v -PE newserver
and get that few port with number 40000 and higher are open|filtered.
When I scan newserver again I get other few udp port open (differnt 
number of port).

When I use lsof -i or netstat in this newserver I get only named and 
sshd work in 22 and 53 TCP and UDP port and exim work on localhost 25 
port. No high open udp port are discovered.

Why nmap show this temporarily open high port.
What can I do?