File Ownership Question

I don't understand why but I have a problem with ownership changes on files. I have an OD share that is mounted after authentication. On that share there is a folder owned by 'admin' and a special group. The group permissions are 'rw'. Users belonging to that group drop files into that folder. The problem is that after dropping the file, the owner is changed to 'admin' and I want the owner left unchanged. I do not have the inherit permissions turned on for the mounted share. Why is the owner being changed and how do I stop it? TIA.