You should not be able to modify that file as 'user' with those permissions. Are you sure that 'user' does not belong to group1 ? have you checked with the 'groups' command?
If i have a folder owned by klaatu:users and enter that folder as 'gort', it's OK because gort is in the 'users' group.
However, if I have a file 'foo' in that folder, and foo is owned by klaatu:staff, then as 'gort', I can read the file with cat or a similar command, but if I do something like 'echo bar > foo' then I will get Permission Denied.
I don't think I would call it a "conflict". Maybe looking at a folder's permissions more as a gatekeeper, through which you cannot pass if you do not have the correct permissions. But just because you do have permissions to go inside and look at stuff, it doesn't mean you can change them or use them.
If you really need to understand it, maybe create two users and create two groups. Log in as the first user and create the test environment:
Code:
whoami
> klaatu
groups
users staff blah blah
mkdir /test
chown klaatu:users /test
chmod 770 /test
cd /test
echo "bar" > foo
groups
users staff blah blah
chown klaatu:staff foo
chmod 644 foo
su - gort
whoami
> gort
groups
users blah blah
cd /test
cat foo
> bar
echo "baz" >> foo
> -su: /test/foo: Permission denied