Setting font size of Label to exactly 12 (WinRT) or 15 (UWP) results in much larger font

In Xamarin.Forms Windows PCL project, if I set FontSize of Label to exactly 12 (WinRT) or 15 (UWP), which is what Device.GetNamedSize(NamedSize.Default, typeof(Label)) returns, the rendered font size is much larger, even larger than of label with font size +1.

Looking into Xamarin.Forms code on git (https://github.com/xamarin/Xamarin.Forms/blob/master/Xamarin.Forms.Platform.WinRT/FontExtensions.cs and https://github.com/xamarin/Xamarin.Forms/blob/master/Xamarin.Forms.Platform.WinRT/LabelRenderer.cs ) it seems the font is recognized as default, but than font size of medium is used, which is much larger.
On iOS and Android default font size is the same as medium.

Example code:

var stack = new StackLayout()
{

    Orientation = StackOrientation.Vertical

};

for (var i = -4; i < 4; i++) { stack.Children.Add(new Label() { Text = "font size " + (Device.GetNamedSize(NamedSize.Default, typeof(Label)) + i), FontSize = Device.GetNamedSize(NamedSize.Default, typeof(Label)) + i }); };


Reference: https://bugzilla.xamarin.com/show_bug.cgi?id=42517