I have a very wierd issue with one single folder in my server shared folders. From time to time, a directory or a file moved under this subfolder won't inherit permissions as supposed to. Instead, it would get a custom ACL, with only permissions for the user who moved the folder in there, and administrators. If I try to edit the ACL (i.e. add a user) and apply the changes then the inheritance is again applied correctly. Did you ever experience anything like that?
When you move a file or folder on the same drive letter then it will retain the original permissions of the file (unless the original was also inheriting). However, if you copy it, then it will assume the permissions of the new folder.
As an experiment, try copying a folder vs. moving it. Also, try moving a folder from a different volume. In both of those cases it should inherit the permissions correctly.
This will hopefully confirm why it appears to be difference in some situations than in other situations.