Short: When you change your network configuration twice during installation only the first (and surely false) one is written to system.

Long: You install - after you let the installer send DHCP requests you're able to provide your IP address information.  If you have a typo or (like me) choose the wrong net, the installer is unable to connect to the mirrors. So I went back to "Configure Network" and provided the right address, net and router. Installation went fine and i rebooted.

First thing after reboot i wanted to install some packages and was very surprised to see, that my very first and false information was written to /etc/network/interfaces.

I hope these are all the information you may need.

regards,
 -AmP