The Problem was that the DataGridView was not Bount to an Source and I was adding rows like this:
private void LoadTableItems()
foreach (KeyValuePair<int,> param in UsedTable.Parameters)
string value = param.Value.Value.ToString();
string name = param.Value.Column.Name;
string descriptionName = param.Value.Column.HeaderText;
name, descriptionName, param.Value.RationalOperator.ToString(),
Because of that the Value of the ComboBox Cell is interprated like the ValueMember and not as DispleyMember. Also because of that the ComboBox works like it sould after I work with the ComboBox. The Solution should be to make a Temporal DataSet/DataTable, add the Parameter Values to that DataSet and then bind the DataSet to the DataGridView.