Xamarin Forms Global Style with Specific Styles

I am working on applying global font styles to my application right now. I do this using the following code:

<Style TargetType="Button">
    <Setter Property="FontFamily">
        <Setter.Value>
            <OnPlatform x:TypeArguments="x:String"
                        iOS="HelveticaNeue"
                        Android="sans-serif"/>
        </Setter.Value>
    </Setter>
</Style>

This works great and styles the buttons with a FontFamily. I am also setting named styles on certain buttons using the following code:

<Style x:Key="BaseButtonStyle" TargetType="Button">
    <Setter Property="BackgroundColor" Value="Transparent"/>
    <Setter Property="BorderWidth" Value="1"/>
    <Setter Property="BorderColor" Value="{x:Static map:Constants.Blue}"/>
</Style>

The named styles override the global styles as expected but I guess I was hoping that they would only override the styled properties that actually overlap (if I was, for example, specifying a FontFamily in both the global style and the named style).

Is this expected behavior for a named style to override properties that are not even being defined? It is a fairly simple workaround to just add the FontFamily style into all of my named styles but I wanted to see what other people think of this. Thanks.