Font Resources Look Different When Using Font Family

I have created a font family for the Arial font as follows:
<?xml version="1.0" encoding="utf-8" ?>






I have also created the following style resource:

In my code, I have the following Button elements:










This displays the following:

I would have expected the first & second 5 Button(s) to look the same, but notice the slight differences, even though they use the same font resources. Why do they look different when using a font family?