When using ItemsControl ItemsControl.ItemsPanel is set to Canvas, ContenPresenter comes in and break my Canvas properties on the children [WPF]

I am using an ItemsControl where the ItemsPanel is set to Canvas (see this question for more background information). The ItemsControl is performing as I want, and it works like a charm when adding a child element manually by putting it into ItemsControl.Items:

<ItemsControl>
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <Canvas IsItemsHost="True" />
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
    <ItemsControl.Items>
        <Button Canvas.Left="500" Content="Button Text" />
    </ItemsControl.Items>
</ItemsControl>

Note the Canvas.Left property on the Button. This works like a charm, and the Button is placed 500 pixels from the left of the ItemsControl left side. Great!

However, When I am defining a ItemsSource binding to a List, the Canvas.left doesn't have any effect:

<ItemsControl ItemsSource="{Binding Elements}">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <Canvas IsItemsHost="True" />
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <Button Canvas.Left="500" Content="Button Text" />
        </DataTemplate>     
    </ItemsControl.ItemTemplate>
</ItemsControl> 

By inspecting the application during run time, I see one difference. The container ContentPresenter has been added between the Canvas and the button..

How can I set the Canvas.Left property on the ContentPresenter itself? Or is there another way to solve this problem?

Thanks to all!