Submitted files are being created with the wrong group (wheel)

I have a user www that is a member of the group www. Both are set in the respective User and Group setting of httpd.conf. Yet the submited files are created using the wrong group wheel.

I checked the cache and public directories and in both of those places the files are correctly created using the group www. Just the files in the files_dir are using the wrong group.

I noticed that this has an effect on my backup routine. How can that be fixed?