Hello, 

This is caused by the order of which the events are fired. E.g.:

         <asp:CustomValidator ID="CustomValidator1" runat="server" ControlToValidate="SelectTest"
                EnableClientScript="true"
                ErrorMessage="Custom Validator" ClientValidationFunction="testValidation"></asp:CustomValidator>

            <select runat="server" id="SelectTest">
                <option value="0">Value 0</option>
                <option value="1">Value 1</option>
                <option value="2">Value 2</option>
            </select>

        function testValidation() {
            console.log("validation");
        }

        $(document).ready(function () {
            $("#SelectTest").bind("change", function () {
                console.log("selected index has been changed");
            });
        });


With the above code you will notice that the validation is fired before the 'change' event which we handle internally in our control.

To overcome that behavior in your sample you should get the new value directly from the select element:
$telerik.$(combo.get_element()).find("select").val().

Keep in mind that after the validation event the value of the RadComboBox is properly returned by the get_value() property. 

Regards,
Peter