Uri.IsWellFormedUriString for relative Hashbang urls compatibility

In the following tests, why does (only) the last one fail?

    [Fact]
    public void IsWellFormedUriString_AbsolutNonHashTagUri_ReturnsTrue()
    {
        Assert.True(Uri.IsWellFormedUriString("http://www.RegularSite.org/Home", UriKind.Absolute));
    }

    [Fact]
    public void IsWellFormedUriString_RelativeNonHashTagUri_ReturnsTrue()
    {
        Assert.True(Uri.IsWellFormedUriString("Home", UriKind.Relative));
    }

    [Fact]
    public void IsWellFormedUriString_AbsolutHashTagUri_ReturnsTrue()
    {
        Assert.True(Uri.IsWellFormedUriString("http://www.w3.org/#!Home", UriKind.Absolute));
    }

    [Fact]
    public void IsWellFormedUriString_RelativeHashTagUri_ReturnsTrue()
    {
        // Fails!
        Assert.True(Uri.IsWellFormedUriString("#!Home", UriKind.Relative));
    }

If Uri recognizes Hashbangs in the Absolute version of IsWellFormedUriString, why not in the Relative version? What am I missing?

Note: This doesn't help.