Do you mean that a person using a Mac opens a database that resides on the server and that person sees a different ACL than a user running Windows? I know of no way that can happen since the ACL lives with the db, not the client.
If something else is happening (like each user is getting some local or server based copy of a db or template and the ACL is different based on the user's OS) can you provide details of how the db is created for the user?