关于RIA Service中删除实体的奇怪问题！

代码如下：
            AccountDomainContext db = new AccountDomainContext();
            db.Load(db.GetAccountsQuery(), callback =>
                {
                    if (callback.HasError)
                    {
                        MessageBox.Show("ReadError");
                        callback.MarkErrorAsHandled();
                    }
                    else
                    {
                        Account a = callback.Entities.FirstOrDefault();
                        if (a!=null)
                        {
                            var r = a.AccountRights.Where(x => x.RightId.Equals(1)).SingleOrDefault();
                            a.AccountRights.Remove(r); //移除实体
                            db.SubmitChanges(savecallback =>
                            {
                                if (savecallback.HasError)
                                {
                                    // savecallback.ChangSet中指示
                                    // ModifiedEntities.Count = 1
                                    // RemovedEntities.Count = 0
                                    // 明明是移除却变成了修改，导致保存失败。
                                    MessageBox.Show("ERROR");
                                    savecallback.MarkErrorAsHandled();
                                }
                            }, null);
                        }
                    }
                }, null);
确实搞不懂了，明明是移除实现操作，可ChangSet中却提示是修改 。。  这是什么情况 ？