LongListSelector centered items unexpected position change - xaml

Question

I have a LongListSelector control on a page in my Windows Phone 8 app. For this control I set a GroupHeaderTemplate and a ItemTemplate and both contain a TextBlock. If I align the TextBlock controls to center I have some weird behavior (maybe not weird but unexplainable to me at the moment). When I open the page on my phone the centered text moves ca. 5px to the right. The move is visible to the user (it's happening after the page is loaded). I tried to solve this problem but all margin changes (and other trials) did not change the behavior and my friend Google couldn't help me either. Can some explain to my why this is happening and how to solve this?
My code (simplified):
<Grid x:Name="ContentPanel" Grid.Row="1" Margin="0">
<phone:LongListSelector x:Name="longList" LayoutMode="List" HideEmptyGroups ="true" Margin="5,0,5,5" ItemsSource="{Binding List, Mode=OneWay}" IsGroupingEnabled="True" Background="Transparent" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<phone:LongListSelector.Resources>
<DataTemplate x:Key="itemTemplate">
<Grid Margin="10,10,10,0" Background="#FFF3F1F1" HorizontalAlignment="Left">
<TextBlock x:Name="name" Margin="10" TextWrapping="NoWrap" Text="{Binding Name}" VerticalAlignment="Center" FontSize="40"/>
</Grid>
</DataTemplate>
<DataTemplate x:Key="groupHeaderTemplate">
<Grid Margin="10,10,10,0" Background="#FFF3F1F1" HorizontalAlignment="Center">
<TextBlock x:Name="name" Margin="10" TextWrapping="NoWrap" Text="test" VerticalAlignment="Center" FontSize="40"/>
</Grid>
</DataTemplate>
</phone:LongListSelector.Resources>
<!--<phone:LongListSelector.JumpListStyle>
<StaticResource ResourceKey="jumpListStyle"/>
</phone:LongListSelector.JumpListStyle>-->
<phone:LongListSelector.GroupHeaderTemplate>
<StaticResource ResourceKey="groupHeaderTemplate"/>
</phone:LongListSelector.GroupHeaderTemplate>
<phone:LongListSelector.ItemTemplate>
<StaticResource ResourceKey="itemTemplate"/>
</phone:LongListSelector.ItemTemplate>
<i:Interaction.Triggers>
<i:EventTrigger EventName="SelectionChanged">
<i:InvokeCommandAction Command="{Binding SelectItemCommand}"
CommandParameter="{Binding SelectedItem, ElementName=longList}"/>
</i:EventTrigger>
</i:Interaction.Triggers>
</phone:LongListSelector>
</Grid>
Thanks in advance