Strange behavior with x! and permissions

I found today the following on a Debian 8.7 (vim 7.4.576) and FreeBSD 11
(8.0.579): I am in my home-dir "user" and there I create a file with the
following permissions: root:user 640

When I open the file as "user", vim tells me that the file is read-only.
I edit it, and close it with :x!
vim writes the file and sets the permissions to:
user:user 640

I didn't use sudo or anything. When I tried to do a chown or chmod on
this file to set the permissions to user:user from root:user it failed
because "user" didn't have the permissions.

What is happening here?

Niels

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