That is because your textboxes gets the value on the client side alone, while the underlying RadTextBoxes values do not.
I have experienced this issue before where values set to a RadTextBox by using jQuery xxx.val('...') do not get through to the server-side on postback.
The only way to get the value 'assigned' to the RadTextBox control's Text property from client-side is to use:
$find('<%= txtUsername.ClientID%>').set_value('newvalue');
In your case, most probably the browsers set (autofill) the values and only available on client side. Therefore, what you can do to actually assign those values is to intercept before the Login button postbacks, like so:
var un = $('#<%= txtUsername.ClientID%>').val();
var pw = $('#<%= txtPassword.ClientID%>').val();
$find('<%= txtUsername.ClientID%>').set_value(un);
$find('<%= txtPassword.ClientID%>').set_value(pw);
If you are using RadButton as the Login button, this can be done on the button's OnClientClicking/OnClientClicked event like so:
<telerik:RadButton ID="cmdSignin" runat="server" Text="Sign In"
SingleClick="true" SingleClickText="Signing In..."
OnClientClicking="OnClientClicking"></telerik:RadButton>
Hope that helps.