""" Although Compute Engine doesn't allow creating an instance with a user-defined local IP address, you can use a combination of routes and an instance's ‑‑can_ip_forward ability to add local IP address as a network static address which then maps to your desired virtual machine instance.

For example, if you want to assign 10.1.1.1 specifically as a network address to a virtual machine instance, you can create a static network route that sends traffic from 10.1.1.1 to your instance, even if the instance's network address assigned by Compute Engine doesn't match your desired network address. """

Meaning they could have avoided conflicts using this mechanism.