Permissions changes on it own

I have a server 2003 and the ntfs permissions are set for not inherited with
group 'A' having full rights, group 'B' having modiy rights with
specifically deny change permission and change ownership, and group 'C'
having Read/Write rights with specifically deny change permission and change
ownership.

A user is a member of groups 'B' & 'C'. He opens a file (.xls), changes the
file then saves it and shazaam!! the permissions change to the user having
ownership of the file and Groups 'A', 'B', 'C', & 'D' are listed with
inherited permissions.

Can anybody help??? I have banged my head enough for several days checke
over permsisions, policies and rights but nothing is evident that woud allow
this.