Why are files in my home dir being created as world-writable despite a more-restrictive umask?

I don't know if it's proper to answer my own question. Editors, please, advise on this if this is not the case. Thanks in advance.

I think I've solved this mystery: the problem was the lack of a default ACL on the XFS volumes. Here's the ACL entry for /srv/backups, one of the directories affected:

# file: srv/backups
# owner: root
# group: root
user::rwx
group::r-x
other::r-x

Whenever I did a "mkdir test" or "touch testfile", it would came up with permissions 777. So I did this:

setfacl -m d:u::rwx /srv/backups

Leaving the ACL like this:

# file: srv/backups
# owner: root
# group: root
user::rwx
group::r-x
other::r-x
default:user::rwx
default:group::r-x
default:other::r-x

Previously there (supposedly) was no ACL, but now there is. I can see the "+" sign attached to the permissions when I do a "ls -l". And magically, now "mkdir test" and "touch testfile" work with the expected permissions:

# ls -l testfile 
-rw-r--r-- 1 root root 0 Dec 20 10:00 testfile
# ls -ld testdir
drwxr-xr-x+ 2 root root 6 Dec 20 10:00 testdir

I don't know why this happens. I guess XFS doesn't like not having a default ACL, and behaves strangely when it happens. Also, I've seen this happen only in CentOS, not in Debian/Ubuntu. Maybe it's related to the XFS version in the kernel, or something like that. No idea.

Anyway, that settles the case for me. Thanks a lot for all the suggestions :-)