WPF Progress Bar Border Thickness - wpf

Question

For some reason when setting the border thickness for a custom progress bar style the border does not retain the same thickness? Why does this happen? The border will ideally be a value of 1. I just set it to 6 so exaggerate the issue.
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1.Themes">
<Style x:Key="{x:Type ProgressBar}" TargetType="{x:Type ProgressBar}">
<Setter Property="Background" Value="{StaticResource GrayBrush5}" />
<Setter Property="BorderBrush" Value="{StaticResource ControlBorderBrush}" />
<Setter Property="BorderThickness" Value="6" />
<Setter Property="Foreground" Value="{StaticResource AccentLightBrush}" />
<Setter Property="IsTabStop" Value="False" />
<Setter Property="VerticalAlignment" Value="Center" />
<Setter Property="SnapsToDevicePixels" Value="true"/>
<Setter Property="OverridesDefaultStyle" Value="true"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ProgressBar">
<Grid x:Name="Root">
<Border x:Name="PART_Track"
Margin="0"
SnapsToDevicePixels="True"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}" />
<Grid x:Name="ProgressBarRootGrid">
<Grid x:Name="IndeterminateRoot" Visibility="Collapsed">
<Rectangle x:Name="IndeterminateSolidFill"
Margin="{TemplateBinding BorderThickness}"
Fill="{TemplateBinding Background}"
RenderTransformOrigin="0.5,0.5"
StrokeThickness="0" />
<Rectangle x:Name="IndeterminateGradientFill"
Margin="{TemplateBinding BorderThickness}"
StrokeThickness="0">
<Rectangle.Fill>
<LinearGradientBrush MappingMode="Absolute" SpreadMethod="Repeat" StartPoint="20,1" EndPoint="0,1">
<LinearGradientBrush.Transform>
<TransformGroup>
<TranslateTransform x:Name="xTransform" X="0" />
<SkewTransform AngleX="-30" />
</TransformGroup>
</LinearGradientBrush.Transform>
<GradientStop Offset="0" Color="{StaticResource AccentLightColor}" />
<GradientStop Offset="0.5" Color="{StaticResource AccentLightColor}" />
<GradientStop Offset="0.5" Color="{StaticResource AccentDarkColor}" />
<GradientStop Offset="1.0" Color="{StaticResource AccentDarkColor}" />
</LinearGradientBrush>
</Rectangle.Fill>
</Rectangle>
</Grid>
<Grid x:Name="DeterminateRoot" Margin="1">
<Border x:Name="PART_Indicator"
Margin="0"
HorizontalAlignment="Left"
Background="{StaticResource AccentLightBrush}">
<Rectangle x:Name="GradientFill" Visibility="Collapsed">
<Rectangle.Fill>
<LinearGradientBrush MappingMode="Absolute" SpreadMethod="Repeat" StartPoint="20,1" EndPoint="0,1">
<LinearGradientBrush.Transform>
<TransformGroup>
<TranslateTransform X="0" />
<SkewTransform AngleX="-30" />
</TransformGroup>
</LinearGradientBrush.Transform>
<GradientStop Offset="0" Color="{StaticResource AccentLightColor}" />
<GradientStop Offset="0.5" Color="{StaticResource AccentLightColor}" />
<GradientStop Offset="0.5" Color="{StaticResource AccentDarkColor}" />
<GradientStop Offset="1.0" Color="{StaticResource AccentDarkColor}" />
</LinearGradientBrush>
</Rectangle.Fill>
</Rectangle>
</Border>
</Grid>
</Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Determinate" />
<VisualState x:Name="Indeterminate">
<Storyboard RepeatBehavior="Forever">
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="IndeterminateRoot"
Storyboard.TargetProperty="(UIElement.Visibility)"
Duration="00:00:00">
<DiscreteObjectKeyFrame KeyTime="00:00:00">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="DeterminateRoot"
Storyboard.TargetProperty="(UIElement.Visibility)"
Duration="00:00:00">
<DiscreteObjectKeyFrame KeyTime="00:00:00">
<DiscreteObjectKeyFrame.Value>
<Visibility>Collapsed</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames Storyboard.TargetName="xTransform" Storyboard.TargetProperty="X">
<SplineDoubleKeyFrame KeyTime="0" Value="0" />
<SplineDoubleKeyFrame KeyTime="00:00:.35" Value="20" />
</DoubleAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="Orientation" Value="Vertical">
<Setter TargetName="Root" Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="-90" />
</Setter.Value>
</Setter>
</Trigger>
<Trigger Property="IsIndeterminate" Value="true">
<Setter TargetName="DeterminateRoot" Property="Visibility" Value="Collapsed" />
<Setter TargetName="IndeterminateRoot" Property="Visibility" Value="Visible" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ResourceDictionary>