ProgressBar.Value strangely affects other element's position - wpf

Question

Unless I'm missing something basic, this appears to be a bug. It is simple to reproduce. Just create a new WPF application project and paste the following in MainWindow's XAML:
<Window x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="100" Width="525"
xmlns:local="clr-namespace:WpfApplication1">
<Grid>
<ListBox>
<ListBox.ItemContainerStyle>
<Style TargetType="ListBoxItem">
<Setter Property="HorizontalContentAlignment" Value="Stretch"></Setter>
<Setter Property="IsTabStop" Value="False" />
</Style>
</ListBox.ItemContainerStyle>
<ListBox.ItemTemplate>
<DataTemplate DataType="{x:Type local:AudioFile}">
<Grid HorizontalAlignment="Stretch">
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="3" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="32" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<Image Width="24" Height="24" VerticalAlignment="Center" HorizontalAlignment="Left">
<Image.Source>
<BitmapImage UriSource="{Binding Icon}" />
</Image.Source>
</Image>
<TextBlock Grid.Column="1" Text="{Binding FullPath}" VerticalAlignment="Center" />
<ProgressBar Grid.Column="1" Opacity=".3" Foreground="LightGreen" Value="{Binding UploadProgress}" />
<TextBlock Grid.Column="1" HorizontalAlignment="Right" VerticalAlignment="Bottom" FontSize="8">
<Run Text="{Binding UploadRate, StringFormat={}{0:00}}" />
<Run> kbps</Run>
</TextBlock>
<Image Grid.Column="2" Width="16" Height="16" VerticalAlignment="Center" />
<ProgressBar Grid.Row="1" Grid.ColumnSpan="3" Value="{Binding PercentCompleted, Mode=OneWay}" />
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>
<ListBox.Items>
<local:AudioFile FullPath="C:\audio1.mp3" Status="Uploading" UploadProgress="0" PercentCompleted="40" ErrorUploading="" />
</ListBox.Items>
</ListBox>
</Grid>
</Window>
Add a new class named AudioFile and paste the following code therein:
Public Class AudioFile
Public Property FullPath() As String
Public Property UploadProgress() As Integer
Public Property UploadRate() As Single = 120
Public Property Status() As String
Public Property ErrorUploading() As String
Public ReadOnly Property Icon() As String
Get
If String.IsNullOrEmpty(FullPath.Trim()) Then
Return ""
Else
Dim RetVal As String = "Resources/speaker.png"
Select Case System.IO.Path.GetExtension(FullPath).ToLower().Trim("."c)
Case "mp3"
RetVal = "Resources/mp3.png"
Case "wav"
RetVal = "Resources/wav.png"
End Select
Return RetVal
End If
End Get
End Property
Public Property PercentCompleted() As Integer
End Class
Now go to XAML and change the value of PercentCompleted from "40" to "80". Note the position of FullPath TextBlock as it jumps to further right. Why? The only binding we have for PercentCompleted is of ProgressBar's Value.