WPF: Why does ItemsPresenter override my DataGrid's foreground style?

Question

I have a DataGrid style in App.xaml:

<Style TargetType="{x:Type DataGrid}">
    <Setter Property="Foreground" Value="{StaticResource DataGridItemTextBrush}" />
    <Setter Property="VerticalGridLinesBrush" Value="{StaticResource GridBrush}" />
    <Setter Property="HorizontalGridLinesBrush" Value="{StaticResource GridBrush}" />
    <Setter Property="RowBackground" Value="Transparent" />
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="HeadersVisibility" Value="Column" />
    <Setter Property="AlternatingRowBackground" Value="#77000000" />
</Style>

This works great for all of my datagrid's in my applications. However, for one of my datagrids, I want to group my rows if a specific column shares the same values. So I use the following on that particular datagrid:

<DataGrid.GroupStyle>
    <GroupStyle>
        <GroupStyle.HeaderTemplate>
            <DataTemplate>
                <StackPanel>
                    <TextBlock Text="{Binding Path=Name}" Padding="3"/>
                </StackPanel>
            </DataTemplate>
        </GroupStyle.HeaderTemplate>
        <GroupStyle.ContainerStyle>
            <Style TargetType="{x:Type GroupItem}" >
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type GroupItem}">
                            <Expander>
                                <Expander.Resources>
                                    <Style TargetType="{x:Type TextBlock}">
                                        <Setter Property="Foreground" Value="White" />
                                    </Style>
                                </Expander.Resources>
                                <Expander.Header>
                                    <StackPanel Orientation="Horizontal">
                                        <TextBlock Text="{Binding Path=Name, StringFormat=Set: {0}}" Margin="5,0"/>
                                        <TextBlock Text="{Binding Path=ItemCount, StringFormat=(\{0\} Games)}"/>
                                    </StackPanel>
                                </Expander.Header>
                                <ItemsPresenter />
                            </Expander>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </GroupStyle.ContainerStyle>
    </GroupStyle>
</DataGrid.GroupStyle>

Problem: Now this DataGrid displays everything correctly based on my DataGrid style, except it displays the text (foreground) as black instead of my style.

Solution: I can fix the problem (though I don't understand why this is necessary) by modifying my ItemsPresenter to either of the following:

<ItemsPresenter TextElement.Foreground="{StaticResource DataGridItemTextBrush}"/>

or

<ItemsPresenter TextBlock.Foreground="{StaticResource DataGridItemTextBrush}" />

Question: Can anyone explain why this happens and/or offer a better solution that will guarantee that my ItemsPresenter does not override any of my DataGrid styles?

Thank you!