Buttons decorated by RadFormDecorator have 16 additional pixels. Issue is reproducible since Q3 2013 and is caused due to the box-sizing and -moz-box-sizing styles. For the time being one of the following workarounds can be used:

1) Override the box-sizing style:

CSS:

    <style>
        .RadForm, .RadForm.rfdTextbox .rfdDecorated {
            box-sizing: border-box !important;
            -moz-box-sizing: border-box !important;
        }
    </style>

ASPX:

        <telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" />
        <div style="margin: 20px; margin-bottom: 0; border: 2px solid Black; height: 15px; width: 168px;"></div>
        <div style="position: absolute; margin: 10px; margin-top: 0; padding: 10px; padding-top: 0;">
            <div>
                <asp:Button ID="Button1" runat="server" Text="Cancel" TabIndex="1" Width="80px" UseSubmitBehavior="false" />
                <asp:Button ID="Button2" runat="server" Text="Save" TabIndex="2" Width="80px" UseSubmitBehavior="false" />
            </div>
        </div>
2) Set the rendermode of the FormDecorator to lightweight:

ASPX:

        <telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" RenderMode="Lightweight" />
        <div style="margin: 20px; margin-bottom: 0; border: 2px solid Black; height: 15px; width: 168px;"></div>
        <div style="position: absolute; margin: 10px; margin-top: 0; padding: 10px; padding-top: 0;">
            <div>
                <asp:Button ID="Button1" runat="server" Text="Cancel" TabIndex="1" Width="80px" UseSubmitBehavior="false" />
                <asp:Button ID="Button2" runat="server" Text="Save" TabIndex="2" Width="80px" UseSubmitBehavior="false" />
            </div>
        </div>

A possible workaround is to avoid the decoration of button controls by setting RadFormDecorator's ControlsToSkip property to "Buttons".