Why are my Windows 8 WPF radio buttons all out of whack?
Everything in WPF is styled. In the case of the default system styles they sometimes, in my own experience, have little issues like this. A simple fix, to confirm it is the style and not the OS itself is to restyle the control yourself and see if that fixes it.
You can find the original style by looking at this question. These will help you rebuild and / or fix the issue