Relation between resource referencing way and sharing regarding Storyboard

Question

<!-- Here x:Shared is modified -->
        <SolidColorBrush x:Key="SolidBrush" x:Shared="False" Color="White"/>
        <Style TargetType="{x:Type TabItem}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type TabItem}">
                        <Grid x:Name="Root">
                           <VisualStateManager.VisualStateGroups>
                                <VisualStateGroup x:Name="SelectionStates">
                                    <VisualState x:Name="Unselected" >
                                        <Storyboard>
                                            <ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
                                                Storyboard.TargetProperty="Background.Color">
                                                <EasingColorKeyFrame KeyTime="0" Value="Red" />
                                            </ColorAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="Selected">
                                        <Storyboard>
                                            <ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
                                                Storyboard.TargetProperty="Background.Color">
                                                <EasingColorKeyFrame KeyTime="0" Value="Blue" />
                                            </ColorAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                </VisualStateGroup>
                            </VisualStateManager.VisualStateGroups>
<!-- Here I change the refencing way of Background -->
                            <Border x:Name="Border" Background="{DynamicResource SolidBrush}">
                                <ContentPresenter x:Name="ContentSite" ContentSource="Header" RecognizesAccessKey="True" />
                            </Border>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>

I wondered how Storyboard works regarding resource referencing way and sharing. So I tested the above code and got a result like below.

Reference way in Border	x:Shared	Result
DynamicResource	True	IsFrozen==True
DynamicResource	False	Cannot Animate Background.Color on an Immutable Object Instance Error
StaticResource	True	IsFrozen==False
StaticResource	False	IsFrozen==False

Debug.WriteLine((Resources["SolidBrush"] as SolidColorBrush).IsFrozen);

IsFrozen is a result of above code.

I found that if resource is referred in a Style it is automatically frozen.(SO, I don't know why frozen). As storyboard clone a frozen resource to animate it, I can understand why DynamicResource works correctly.

But I cannot understand why a resource is not frozen in case of StaticResource, and why referencing non-shareable resource causes Cannot Animate an Immutable Object Instance error.

Why these happen?